Everything Mrlizard said is true. Believe! I would recommend against groove tubes though man, its just a name and they aren't all they are cracked up to be, claiming they are precisely matched and actually being quite far out. Buy a matched set of power tubes from a amplifier technician, not a shop. They often don't have a clue! And Paul is right too...JJ EL34's are sweet as are their ECC83's, but buy a few different preamp tubes and experiment...if you can find mullard NOS for a good price then get em...the best ECC83's ever.
I have just had my 1959 retubed with Electro harmonix EL34's and JJ ECC83's with and it sounds REALLY good
oh yeah...if you are lacking clarity that was there before, it could be failing filter caps. Man...i love them, but valve amps are pretty confusing...