Hello.

I have an OTS 100W. In my opinion and with my particular amp, the best loud cleans that are "really clean"  are with volume at 6-7 and master at 3,5. Beyond that, it begins to breakup a little, but it is very loud at this point.
I think that the bias is very important for this too. Mine is at 32mv. The higher, the earlier breakup.

Hope this helps.

Nop. I plug my guitar ('68 Guild Starfire V) directly to the amp with nothing in the loop. This is my setting for my jazz fussion trio. I think that the buffer have nothing to do with the clean headroom. This is only my opinion cause I'm not an expert.

I also have my guitar plugged in to the norm, which would be better for clean, norm or Fet?

That amp is not as tolerant as my HRM for pick attack on the cleans at higher clean gain settings.  The bias thang is important.  the gain stting that works for your apartment will be too high for playing out.  Drop it a full number at gig volume and compensate with the master.  Since you are mainly playing clean, you can sub out a lower gain tube at V1.  Vintage moderately used GE 5751 or Sylvania 12at7/6201 tubes will allow higher gain knob settings for the same amount of gain and sound better tonewise for cleans than the stock JJ's there, while not as affordable as new production tubes, among vintage old stock they are excellent value starting point, more than worth the cost and effort of procurement of what needed plus spares/flavors in different VOS tube makers.

Ratio knob...go to Ceriatone.com and find the manual for your amp.  You will be glad you took the time to find it and read it!

Yeah, i was thinking the same thing. If i recall the standard OTS stays cleaner longer than the BMs when on the clean side, and mine stays clean pretty far up and less i whack it with boost