I've had my ultras for over 4 years when all of a sudden the sub would begin cutting in and out with some popping (sats were fine); the problem was definitely worse on hot days. Then I remembered back in 2002 I had a computer video card with similar behavior, i.e. it would work fine in 3D mode for a couple of minutes and then quit. The problem was defective capacitors on the card. After opening the subwoofer I found a small capacitor (22 mf 50v) on the power supply board that had leaked dark brown goo. At first it was a little hard to tell the leaked capacitor goo from the yellow-brown glue used to keep some of the larger components from vibrating. Googled "bad capacitors" with the manufacturers' names and discovered that the electrolyte they used was allegedly missing some key ingredients causing them to 'vent' their electrolyte when stressed/heated. One thing to keep in mind is that capacitors can fail without any outwardly visible signs like a buldging can or vented electrolyte.
I am not an electrician, but am fairly handy so I did some research and ordered replacement electrolytic capacitors with higher temperature tolerances for the entire power supply board as well as a more robust (2 watt) replacement resistor (R529 - 5110 Ohm I think) that was severely discolored. Yesterday all the parts arrived and I tried my hand at soldering for the first time. The capacitors have a positive and a negative lead that must be installed in the correct direction, so I noted their orientation before removing each one.
Everything sounds great - I am completely floored.My one concern is that while they were failing, the capacitors were not doing their job and may have degraded other electronic components in the amplifier. The resistor that looked burned up was connected directly to the leaky capacitor in my system. The most useful sites for trouble shooting that I found were,
1) http://cgi.ebay.com/COOLING-KIT-UPGRADE-for-Klipsch-ProMedia-5.1-THX-Ultra_W0QQitemZ330330032034QQcmdZViewItemQQimsxZ20090514?IMSfp=TL090514145014r35085
2) http://www.ocforums.com/showthread.php?t=502498
Excellent schematics for the 5.1 Ultra amplifier are at:
3) http://www.thompdale.com/bash_amplifier/bash_amp.htm
Information on bad caps can be found at:
4) http://www.badcaps.net/forum/showthread.php?t=388
5) http://my.execpc.com/~endlr/index.html
One thing I may try next is to construct a simple stand-off frame with vents in it that would attach to the hole in the sub that the plate amp sits in now. The plate amp would then screw into the stand-off. This would allow a significant amount of heat out of the sub and hopefully extend the life of the amp. It would also be completely reversable if it changed the sound characteristics dramatically.