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Step 3Buying a vintage amp 'project'
Before you buy a vintage amp, do your homework. Check for schematics here: schematicheaven. With any luck, you'll know loads about the amp before you even purchase it! (don't make a silly mistake and buy an old solid-state amp off ebay, thinking that every amp made in the '60s was tube!)
I'd recommend hands-on--i.e., play the amp before purchasing. But there are so many vintage amps on ebay, etc., that's not always possible.
Also, read some user reviews here: Harmony-central Amp Reviews
Things to look for in a vintage amp:
--Does it work at all? This could be a recipe for frustration--if the transformers are bad, you could be looking at a very expensive project (of course, you might have a bargain, too.)
--If you plan to rebuild yourself, simplicity is good. Are you prepared to tackle an amp with tremolo and reverb circuits, as well as a multi-stage amplifier circuit? If not, stay small.
--If modding is your goal, then point-to-point wiring is best (most vintage amps are wired this fashion, anyway.) Unlike a modern PCB, point-to-point is a rats-nest approach, where the components are simply tied together with lengths of wire. This is a good thing, and actually makes noise-related problems easier to fix.
Things to avoid:
--Unsafe design
Look for at least two transformers--every tube amplifier has an output transformer hooked to the speaker. But some old amps forgo input (power) transformers, and are wired directly (indirectly, actually, through a single cap) to the AC line voltage. This type of wiring lacks 'isolation' and is dangerous! If that one old cap fails, you're a conductor! (in the electrical, not the musical sense.) Check online schematics if you can't inspect the amp personally.
The Kay 503A is a bit of a hybrid. It has an isolation transformer--but only for the amplifier circuit, not the tube "heaters." This is not ideal, but it is safer than no isolation at all. With a bit of rewiring, it's not too bad. A separate power supply for the heaters would be the best option, however. Or an external line isolation transformer (replacing the AC heater circuit with an isolated DC supply will also reduce hum.)
--Check for blown speakers or torn cones.
--For a first-timer, avoid a large complex rebuilding project.
I'd recommend hands-on--i.e., play the amp before purchasing. But there are so many vintage amps on ebay, etc., that's not always possible.
Also, read some user reviews here: Harmony-central Amp Reviews
Things to look for in a vintage amp:
--Does it work at all? This could be a recipe for frustration--if the transformers are bad, you could be looking at a very expensive project (of course, you might have a bargain, too.)
--If you plan to rebuild yourself, simplicity is good. Are you prepared to tackle an amp with tremolo and reverb circuits, as well as a multi-stage amplifier circuit? If not, stay small.
--If modding is your goal, then point-to-point wiring is best (most vintage amps are wired this fashion, anyway.) Unlike a modern PCB, point-to-point is a rats-nest approach, where the components are simply tied together with lengths of wire. This is a good thing, and actually makes noise-related problems easier to fix.
Things to avoid:
--Unsafe design
Look for at least two transformers--every tube amplifier has an output transformer hooked to the speaker. But some old amps forgo input (power) transformers, and are wired directly (indirectly, actually, through a single cap) to the AC line voltage. This type of wiring lacks 'isolation' and is dangerous! If that one old cap fails, you're a conductor! (in the electrical, not the musical sense.) Check online schematics if you can't inspect the amp personally.
The Kay 503A is a bit of a hybrid. It has an isolation transformer--but only for the amplifier circuit, not the tube "heaters." This is not ideal, but it is safer than no isolation at all. With a bit of rewiring, it's not too bad. A separate power supply for the heaters would be the best option, however. Or an external line isolation transformer (replacing the AC heater circuit with an isolated DC supply will also reduce hum.)
--Check for blown speakers or torn cones.
--For a first-timer, avoid a large complex rebuilding project.
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The "solid state harshness" you're speaking of is usually thought of as odd-order (3rd, 5th) harmonic distortion, and the "tube warmth" is even-order (2nd, 4th) harmonic distortion. But good tube amps (especially for metal) have both--the odd-order adds bite to the sound.
Pure solid state amps can't produce even-order distortion: transistors can't do that. And the power-amp stage of most high-output (push-pull) tube amps don't produce even-order either--only single-ended power-amps can.
Much of the overall sound of the amp comes from the preamp, which is where a push-pull amp gets it's even-order distortion, plus soft clipping, etc.. Unfortunately, it's not that simple, either. Overdriving the power-amp also adds a particular flavor of distortion that just a preamp can't get.
Adding a "power" tube in the middle of the signal chain doesn't quite work. Power tubes don't amplify voltage much, they amplify current. I.E., they take the low-current signal from the preamp, and convert that to a high-current signal that can drive the speaker coils. What happens if you add a power tube in the chain that doesn't drive a big load? Not much, it just becomes a low-gain preamp tube....
And replacing the power-amp stage alone isn't gonna change the fundamental sound of the preamp.
Plus replacing the S.S. power-amp entirely requires a full-size high-voltage power transformer, an output transformer, expensive tubes, etc. If you've gone that far, you might as well build the tube preamp from scratch, too--you've already spent 75% of the price of a complete tube amp....
I'm not saying you couldn't add some circuitry, but it's a little like trying to build a Hummer from a Ford Focus...by the time you're done, not much of the Ford remains....
I don't suppose you've tried running the output through a 4x12 closed cabinet? That might give you more fullness...
Regardless, everything can be modded, but is it worth it? Right now Crate is selling off all it's current crop of tube amps, like the V50 for $200 (at least the V18 gets good reviews, but you'd probably want more wattage.) I couldn't build one for that price (the tubes alone are probably 1/3 of that price.)
RE: your crate -- You can also try to increase the headroom with a lower mu tube like the 12AV7, 12AT7 or 12AY7. Maybe that, in combination with a front-end pedal like a tube screamer would get you more dynamics and better tone.
Note that I have NO idea if that would make a difference with the Roc Pro, since the tube amplification factor might have no effect on it's use a clipping diode.