If you're wanting to tighten up the bass response, look into some EL34's. They will definitely tame down the bass response compared to the EH 6L6GC's. Here are some comparisons to your current power tubes:
SED EL34's - The bass isn't nearly as deep, but it's definitely tighter. Compared to other EL34's, the bass is still a tad bit soft/loose, but you will not notice this coming from EH 6L6GC's. There will be more mids and the highs will become brighter and maybe a little more detailed. These have a real swirly harmonic content when pushed.
JJ EL34's - These have a deeper, tighter bass response than the SED's above, but they don't quite have the same amount of mids and the highs aren't quite as bright to me in a Recto. They're still very good tubes and are extemely robust. These will add a bit more aggression into the power section. The harmonic structure isn't nearly as swirly as the SED's. It's just a straight ahead vibe with aggression. Compared to the EH 6L6GC's, they won't quite have the same amount of low-end, there will be more mids and the highs will probably still be a bit more aggressive. You will also see a JJ E34L. That tube has a deeper low end than the regular JJ EL34, with more headroom. I don't like the JJ E34L because it's always come off as being a harsher sounding tube. The mids seemed harsh and the highs did as well. They might work out for you in your amp.
EH EL34's - These have a deep, tight low end that's kind of like the JJ, yet not quite as deep. This tube has more of a mid hump than the JJ. The highs have a good amount of spread and clarity. Overall, this tube tends to be a little more on the compressed side when compared to the JJ and is just a tad bit more compressed than the SED EL34's, but don't let that steer you away because it's as smooth as butter. You'll notice tighter low end, more mids and smoother highs.
Ruby EL34B-STR's - These tubes aren't quite as deep in the low end as the JJ and may not be quite as deep as the EH, but what they lack there they make up for in the warmth and roundness department. The mids are even with the bass and the highs are smooth. This is probably the warmest sounding EL34 in current production.
If you have to have 6L6GC's....
SED 6L6GC's - These have a tighter low end than the EH 6L6GC and there are more mids and sweeter highs. These tubes sound very similar to the old Sylvania/Philips 6L6GC/7581A power tubes, but they actually have more harmonic overtones and are a little dirtier sounding. These sound great in the Rectos if you really want to keep the 6L6GC vibe.
JJ 6L6GC's - These tubes sound pretty good in the Rectos. If you pick some of these up from www.eurotubes.com, request a #41 or #42 grade because they will get you closer to the Recto's original bias current readings. The low end is super deep and super tight, the mids are nice and the highs are nice. These have more of an aggressive vibe compared to the SED 6L6GC and their harmonic structure isn't as smooth (it's more gravely). These suckers are warmer sounding than the SED's and they're also very robust for a current production 6L6GC.
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