I have two SFPRs (a '71 w/ 12" Celestion Gold and a '75 w/ a Weber 10F150T), neither of which suffers from lack of or loose low end or lack of warmth.
If you were saying "amp is not loud enough", I might suggest selling and buying something different, but that's not the case. Before dumping a fine amp, in this order, I'd:
Make sure it was properly serviced - all caps etc. up to snuff
Bias it a bit on the "hot" side
Speaker swap . Though expensive, a Celestion Gold 10 might do wonders, as might a number of alnico and ceramic options as per above recommendations
Rectifier tweaks as per above
Have tech "BF" amp by changing a few cap values in key spots
I would bet some combination of the above would get you where you want to go. If not, then you've done your due diligence and can move the amp on knowing you gave it every chance.