Audio Research SP8 MK II Sounds

I am an authorized repair station for ARC and a past owner of an SP-6B. I have also owned a McIntosh C-20. They are very different sounding preamps. The C-20 uses a tube rectifier and so the bass can be a bit soft. The top end of the C-20 can also be a bit soft due to circuits like tone controls and a limited frequency response etc. It does not have a flat frequency response. The SP-8 uses a tube regulator in its power supply. It has a much better regulated power supply in it then the C-20 which has virtually no regulation at all. It has a flat frequency response. Both preamps high level stages are similar. The phone stage is a bit different. The SP8 should sound more open, have better bass and have better top end, as well as lower noise. It's possible the preamp needs to be recapped and your regulated voltages are wrong. There's no need to update this unit, which I have done in the past, without first checking to see if the power supplies are running properly.