Audio Research Gain Settings on Line Stage Amplifiers

I have a Audio Research LS 25 Mark II line stage preamplifier and I have had the Gain set on the medium setting all the time, frankly I have not changed it since I bought the Preamp. running mono blocks class A SS amps. I recently purchased a Luxman SQ 3600 Tube Amp which is running around 30 watts per channel (the amp was converted to KT88's) which has JJ brand tubes. I am switching amps depending on the music I am thinking about for that particular session. Tube amp for jazz and acoustic music and the solid state for rock. I was curious about gain settings on the preamp, which others might use with their tube amps, and the output settings on their tube amps, I currently have my 3600 tube amp outputs (both) at about 55% or just over half level. I am not having any trouble driving a moderate sound level with my speakers using the medium gain settings on the preamp, and the 55% on the outputs on the tube amp, but I am at the about 60% volume and it seems to top out at around 70% volume. Speaker sensitivity is about 87db with no chance of changing the speakers in the near future for a more sensitive speaker to accommodate the tube amp.   

Any recommendations on these settings and what experience can you share to help get the most of the SQ 3600? Will I shorten the tube life significantly by upping the output settings on the tube amp or upping the gain on the preamp?  

Other Bits-I use a Sansui SR-525 Turntable with a Nagoaka MP 300 Cartridge and tube preamp for vinyl, or the Gustard A26 Pro Dac and JRiver or the Audiolab CDT for digital. Final sound is from Wilson Benecsh Square 2's (87 db sensitivity) and 2 REL subs.