I have 5 - 20 uf 500 vdc caps in there now and there's a noticeable volume drop
Dude. this and your original question makes me think you shouldn't be working on your amp.
A cap that is over voltage can explode...
That's not how it works. The voltage marked on the caps is maximum voltage that can be put on 'em before they meltdown.
Connecting them the way you describe would indeed create a 22uF cap, but you would also be putting ~400V across a 25V-rated capacitor. Your research is correct - you do not want to do this.
Assuming these are in parallel you have created a 100uF 500V capacitor, which is too much capacitance.
Try a single 20uF 500V - that is within spitting distance of the original cap.
If that doesn't work, then I suggest you look very closely at your workmanship firstly. Generally one would not replace only a single EL cap.. Have you done a complete cap job or other work to this??? I suspect there's more to the story
I simply replaced 5 of the original early 60's 20uf 525vdc filter caps with Sprague ATOM 20uf 500vdc.......and I replaced the bias circuit cap from the old original 25uf 50vdc with the same value Sprague cap...
amp is now about 3/4 of the volume output than before I made these changes...
I think the OP simply doesn't know the proper questions to ask.
Maybe you should have started with "I just made X changes and now I am at 3/4 power, what do I look for?"
If you want help I suggest:
-Tell us the amp and model#
-Give us your DC voltages on the power tube sockets
-Pics help A LOT
You need to start at the beginning...
I have 5 - 20 uf 500 vdc caps in there now and there's a noticeable volume drop.