I live in a small apartment and needed a desk for my living room. It had few requirements:

1) Has to look nice
2) Suitable for both work and entertainment
2) Needs to house all asociated devices
* printer
* cable modem
* WiFi router
* external sound card
* AV reciever
* Subwoofer
* PC
* work related junk (paperwork, pens etc.)
* small multimedia computer (in the future)
3) Few or no visible ugly wires

So I sat down and designed one..

Step 1: Subwoofer Design

To sum it up in three words: you can design a subwoofer that is
a) small
b) sounds good
c) inexpensive - pick any two.

If you can hide the subwoofer, you can make the box bigger and use less expensive components without compromising sound.

Designing DIY subwoofer is actually easier than it sounds. There are a many forums online that have a lot of great information.
Get yourself a computer programm for box design - there are many available, some of them are free (WinISD Pro, UniBox, BassBox etc.)

I used UniBox, which is a free MS Excel spreadsheet. It's free, easy to work with and double checks entered parameters for errors. Get it here:
http://audio.claub.net/software/kougaard/ubdwnld.html

These programs need to know the drivers Thiele-Small parameters in order for them to work. Most respectable manufacturers publish them.

Enter the asked parameters in the spreadsheet and it will offer "standard" design for closed box, vented enclosure, passive radiator enclosure and bandpass. Of course your actual design might be different because of your room accoustics, personal preferences or size constraints.

Time to start searching for that perfect driver...

Step 2: Selecting Subwoofer Driver II

For a moderately sized room I would suggest searching for 10" - 12" subwoofer driver. 8" drivers don't go low enough, 15" drivers while impressive need a huge box to work properly. There are exceptions to the rule, however.

Pick a store that stocks these drivers and start entering the driver parameters in the your box designing program. You will soon find out, that if a driver requires a small box it is also probably expensive and needs a lot of amplifier power because the sensitivity of these drivers is lower. And vice versa - inexpensive drivers often have good sensitivity and dont need a powerful amplifier, but the box size can be quite large especially if you choose the bass reflex alignment (which I strongly recommend). Also notice that for the same port tuning frequency the port lenght increases if the box size is smaller.

IMHO for home subwoofers the sweet spot for box size is around 90 liters. There are plenty of cheap drivers that go low enough and loud enough with just 100W of power and don't need exotic port sizes or passive radiators.

More often than not you will choose "vented" a.k.a. "bass reflex" design. It sounds very good if designed properly and is easy to make, is reasonably efficient, extends the low end response of the driver, improves power handling, there is a good margin for error for the newbie. Closed box systems are better for cars, passive radiator ir best for small boxes that need to be tuned low. Bandbass enclosures are tricky and not in the scope of this instructable. As always YMMV.

To illustrate this I compared Peerless 830668 and Seas L26ROY. Make no mistake - the Seas will blow the Peerless out of the water with the matching passive radiator and 500W amplifier but the Peerless offers great bang for your buck at much lower power levels.

Step 3: Selecting Subwoofer Driver III

After long searching I finally found Peerless SKP254 subwoofer at a bargain price, that satisfied all my requirements:

http://www.ebay.de/itm/Peerless-SKP254-10-inch-hifi-bass-speakers-woofers-PAIR-/230532889775

Unfortunately it is not available anymore but there are plenty of alternatives.

I used 91 L box size and 33 Hz tuning. The simulated response will be down by about 3dB @ 35 Hz and by 10 dB @ 25 Hz. This sounds about right in a moderately sized room since the walls will significantly boost the lower frequencies. Don't worry if your subwoofer isn't flat to 20Hz since most of the fun is at 35-60 Hz anyway and very little musical content is below that.

Don't forget to watch out for cone excursion (choose drivers that have adequate Xmax) and port air speed. If port air speed is too high you need to use longer and larger diameter port.

Step 4: Choosing Amplifier

There are plenty of amplifier options available. Just search for "subwoofer plate amplifier"

Since I used efficient driver, 100W was more than enough and there are many inexpensive options to choose from.
The amp I got was even bigger bargain: I got an used amp which was pulled from Polk audio PSW 110 subwoofer.

Step 5: Designing the Box

The actual box dimensions don't matter much as long as the internal volume stays the same. I chose to put the subwoofer driver at the back of desk to protect it from kids, feet flying objects etc.

I chose 18mm laminated particle board for the construction of desk so good internal bracing is paramount.
I used Trimble Sketchup for designing the desk. For simple projects such as this it takes just a few hours to learn.

Step 6: Other Stuff

After the subwoofer was drawn, the rest of the components just fell into place.

Reciever sits directly on top of the subwoofer because thats were the subwoofer amplifier lies.
Above the reciever is the place for printer. Originally I wanted to load the paper from the desk surface but that didn't work out.

Modem, WiFi router and power sockets are in the middle part of the desk.
PC and drawers will be on the right.

Step 7: Putting It All Together

After the design was ready I decided to order it professionaly made. Unfortunately I don't have the tools nor the workshop to make it myself.

After three weeks my desk was delivered in four pieces. Then I spent whole day putting it all together.

Since this is a laminated particle board construction, there will be some seams in the subwoofer constructions. Fill these with silicone or other sealant. Fill the enclosure with stuffing from 2 or 3 pillows, then screw the amplifier, bass reflex port and subwoofer driver in place.

Then glue the rest of components in place and route wires.

Step 8: Enjoy!

Enjoy the result!

I have been using this desk for a few weeks now and it's a pleasure. It packs so much features yet looks minimalistic.
Subwoofer works really well and the whole desk costs less than subwoofer of similar caliber in the AV store.

This is my entry in the Instructables furniture contest. I'll be glad to answer your questions