You have a Super Reverb and don't know if it's a 4x10?
I have never seen one that wasn't.
Anyway, the Super Reverb is not an amp known for headroom, quite the opposite actually. If it is really a '65 then it's worth quite a bit of money. Just sell it and get yourself something that has the tone you want. There are many discussions on this board about appropriate amps for Grateful Dead tone. For what a real 65 Super Reverb would bring, you can get a Sarno preamp, nice power amp, and Hard Truckers cab.
"His job is too shed light, not to master"