My system is where I want it be and it sounds awesome...

Mc, I'm gonna have 8 subs:)  

A man's got to know his limitations.

I agree with the two subs. Or even one good REL sub since you are using single ended. My speakers will play down to 25 hz and I still have two subs. I just bought a new car. One had a subwoofer option and the other did not. The sound is night and day. SVS will let you return your subs if you do not like it. 2- SB 2000 pros will probably blow your mind. If not they take them back. 

Have GE Triton One.R. Similar to your GE Reference. You don’t need a sub!
 However —- that Oppo ...
I upgraded to an Oppo 105D with full Modwright mod, including tube power supply and tube dac.  Wow!!!  Difference not subtle. Try it if you can.
Get back on that train.

Btw
if you tube roll, start with the preamp. The Carver amp should be just fine with the original tubes.

The 275 amp comes with either Electro Harmonix or JJ tubes for small signal tubes..  I changed to NOS Telefunken for both the 12au7 and the twin 12at7's.  Big difference.  There are quite a few NOS tubes that will make a big difference in that amp.  Sylvania goldpins, Amperex, etc. are all nice change in the signal tubes.  The output or power tubes are all good to go.  Those KT120's are great, although you can run KT88's, KT150's, or even 6550's.