I dont have a 5751 handy, but I did put a 12ay7 in and that was interesting. But, I need a mid heavy tube and all I have are old RCA (bright) and Tung-Sol (top and bottom). I am thinking of calling my NOS tube guy and getting some tubes that will work better for the amp, but for some reason this amp doesn't respond to fancy tubes the way my Tweed 5e3 Deluxe does. My current production Tung Sol 6v6s are making funny noises and the new ones I bought are terrible!
I was looking at speakers as well and was sad to see that the Celestion Greenbacks are now made in China
But, I guess there are a lot of USA made copies you can buy fully broken in which might suit me (and my neighbors) better. Currently I have the Celestion G12H30 55hz made in England speaker. Maybe I just need a tubescreamer
I do have to wonder if it is just me and the huge amount of stress lately as my rig sounds very different every time I plug in. I have spent the little time I have to play trying to fix my board and amp/tubes and I need to just play despite the sound.
It also has me thinking about smaller amps verses larger amps as I played a Mesa Boogie MKIIB for 25 years and I I am used to having the big iron pushing back and the extra bass. I also missed out on a Swart SST 30 that went for a little more than my AST PRO went for!
The main reason I sold the Boogie was I couldn't lift it safely anymore.