Boogie Triple Rectifier: 6L6 vs EL34

Nahh, just use 6L6, they have higher headroom and are clearer sounding than the EL34s in that amp. Unless you only intend to play at bedroom volumes, then EL34's might be more your thing. But then again I'd probably tell you that you got the wrong amp...
 
Generally, EL-34's will have a subtle roll off on the highs and slightly less extended low end. A little bit more mid punch and focus. If you're trying to get less of a mid scooped sound, they can help. If you're looking for a thicker tone, stick with 6L6's.
 
I really dislike 6L6s in 3 Channel Rectos, though I still prefer EL-34/KT-77s in 2 channels as well. EL-34s make them significantly tighter and clearer in the midrange. KT-77s also sound fantastic in them, since they have the chunky lowend, with the tighter feel of the EL-34s.
 
Are there any mods that tighen up this amp? I think that is the amps biggest problem, it is to loose sounding for most projects.
Is a tube screamer not enough? Also, are you keeping the bass pretty low (around 9:30)? If that isn't enough, maybe try a different cab and/or possibly EL-34's.

Another thing you can do is run an EQ pedal after the tube screamer to give more tone shaping and revoice the amp a bit. This works well. Set the EQ in a "frown" pattern with the mids boosted slightly and the highs and lows cut slightly. It doesn't take much and you can get a pretty good punchy tone that's more mid focused.
 
i prefer 6l6's with a tube screamer and a mesa oversize cab. you will find people that prefer el34's. they will give you less high and lows and more mids. better to try them out and decide for yourself.