Bassman Micro FX loop

[TacosSupreme]

Hi all!
I recently built a bassman micro with cathodyne phase inverter based on Rob Robinette's design.
It's fantastic and might be the perfect amp for home use.
When using the passive fx loop to plug a reverb pedal I can't quite put the preamp volume as high as I'd like to get that sweet edge of breakup sound I love, otherwise the power amp tube starts distorting and not in a good way (or at least I think that's what's happening).
So I'm wondering if that's just a limitation of the design I have to live with or if I could try inserting one of those "zero loss fx loop" circuits you can find around on the interwebs. Not even sure that would solve my problem but my understanding is that the purpose of the circuit is to "normalize" input and output level. There's not that much info available on those circuits it seems, and they are mostly used to mod Marshall amps.

Thanks

[CascoSieg]

It would be helpful to see a schematic of your build, including the fx loop.
Thanks!