I think you can’t compare stereo and mono Frankensteins. The maximal power is not the only important thing. Is even much more important that stereo and mono have different driver tubes and power supplies.
I don’t have Frankenstein amplifier but I have my DIY 300B amplifier that I did dozens changes and tweaks. I can say, for example, changing powers supply capacitors value, type or model can make more impact on sound than changing of output transformers. Or increasing current of driver tube can change sound dramatically. The amplifier with weaker driver, smaller and sheared between two channels power supply can’t sound similar to monoblocs.