Altec speakers can be pretty bright. I have a 15 inch Altec 418b that has a wonderfully detailed top end, but I run the treble knob turned down a bit for the dirty channel if I play through it. An amp as old as yours is has a strong probability that some of the resistors and caps in it have drifted from original values. I would definitely take it to a tech and have the bias checked, and at least change out the bias resistor if bias does not fall into the 60% max plate dissipation range +- 5% for those GE's when installed. But putting in the adjustable bias mod would most definitely be the way I'd want to go if I owned one. I'd also pimp the pre with some vintage tubes from the era that it was made, like some shortplate Sylvania, Mullard, Amperex, or especially Siemens 12ax7's. If you have the opportunity to purchase some decent vintage 6l6gc, by all means do it, why cheat a masterpiece out of the opportunity to be its best? They will sound warmer and better overall than any modern 6l6gc in that amp. The preamp tubes will have an even greater effect on the tone than the power tubes. The speaker has more effect than tubes, though.