-I don't think any EL34 or variant is gonna tighten up your tone, tube wise a EH-6CA7-(a true 6CA7)-would be a better fit, not as compressed, bigger envelope for more head room, or KT77's,
Unfortunately nothing will... is this one of the original 6 knobbers? With the bass pull? What i did for mine was gut it and put a PTP 2204 circuit in. Besides that, you can set the bass knob to zero and pull it. It still makes a difference without all the mud. Ultimately that amp needs to be played very loud to even it out.
I have one of those and the low end is nice and tight. I do run it at least at 4-5 on the master though with an attenuator. I don't use the bass boost but if i do it's still tight, just very big. Maybe the filter caps need replacing?