Hiho,I have used an OTB500 for 5 years now with a SP212 and anOBC115.As youve found out your never short of volume.
Here is almost flat setting which works
Treble-12 o'clock.Middle'12oclock.Bass-9 o'clock.
Another little tip is to put a BDI21 in front of amp.I got mine S/H a decade ago,still works perfect.
Even after 5 years I still get excited playing through this.
To recap get your head round the EQ and your there. My one has JJ Ecc83s valves as standard and DI works perfect.