A good mod might be changing the frequencies of the shelving filters. HF might be no problem, just changing R140 and/or C122 on the Input PCB. The LF section I don´t understand. It looks like a gyrator with a transistor instead of an OP. But it´s just a guess since I´m a jerk in audio electronic. Maybe someone else can help here. As a guess it should be changing C123 for getting another frequency.
Another thing is leaving phantom power on all the time. This gives the right polarization voltage to the coupling caps. This might be an easily achievable improvement (if you do use the Mikepres). But there is reason not to use them ;-). MAybe changing to another OP (U101) can help. IIRC it´s a 4558 which is not really considered to be the best in audio application. Check the metas about changing OPs or this link:
http://www.tangible-technology.com/audiobasics/upgrades/upgrade_2_opamps.htm
Maybe you have to work the PSU over since other OPs draw higher currents.
:sam:
Jens