This thing sounded pretty good out of the box. For $399 i couldn't pass it up. I had heard it was a JCM 800 clone, which it really isn't, but A/B'd against a real one it was pretty freaking close. The JJ 12AX7s are great for low end, and while i'm sure a more suitable replacement could be had for the Ruby EL34s, they still do a pretty good job.
While it was holding it's own at practice, the low end was a bit lacking as usually is with guitar amps. I figured there HAD to be a way to get more low end out of this. For under $25 you can get the parts to do it, all it requires are just a handful of caps.
The biggest single thing you can do to improve the low end is to replace C33 with a 10nF (stock is 22nF). This cap controls the bass tone control. If you do nothing else to the amp this one mod will give you noticably more bass. This mod increases the amount of bass boosted and moves the corner freq of it up some to include more of the low end. Keep in mind the tone controls are interactive, i've found that cranking the mids and keeping the treble at about 1/2 or below gives the most bass.
To get the full effect though, you need to let the low end through the preamp stages, and then out to and through the PI. As the gain builds in the preamp the low end is progressively cut out. For ease of modification, i suggest piggybacking a 0.1uf 400v cap onto C3, then 1uf 400v caps onto C18, C6, C13, C17, and C7. I tried putting 1uF caps on the first two gain stages as well (C39 and C3) but the amp became unstable.
Piggybacking sums the value of the caps so you get both values added together. I found this the easiest option because the leads on the 1uF caps are quite large and did not fit easily through the existing holes. Plus if you do a good job soldering and keep the leads short there's not reason not to piggyback them. They've been this way on my amp for over 2 years with no ill effects.
C17 and C7 are on the power amp board and will require you to remove it for access. The others can be tacked on without removing the preamp board but you'll have to remove it to replace C33 anyways.
This turns a decent amp into a firebreathing bass monster. It'll do a good growl but it's really a one trick pony meant to have the preamp gain cranked. For a few hours work you can get a great sounding amp that doesn't need a distortion pedal- plug and play and you're good to go. Even better yet, i've seen them go used for a little as $200!
Sound samples? Check the first two songs on our profile- recorded with an Ibanez SR405 through an ampeg 8x10 and no pedals or effects. www.myspace.com/dissolute
If you need the schematic there's lots of them floating around the net.