biamping with 75 watt solid state and 18 watt tube amp.klipsch rp600m...t

"Get amps with a pair of balls.
18 watts? Really? That’s like baseball with a whiffle bat.
18 watts, 75 watts,..........this is your problem! "

the OP is using a pair of Klipsch bookshelf speakers and they have a sensitivity of 96 dB.....so a amp with 75 wpc is more than enough.

so this is not his problem.....his problem is something else.

OP needs to check their email.

"...the OP is using a pair of Klipsch bookshelf speakers and they have a sensitivity of 96 dB.....so a amp with 75 wpc is more than enough..."

Except Klipsch efficiency ratings are a bit optimistic and it's a 38 wpc amp into 8 ohms (maybe).  

lets start with the specifics of what you already own.

IMO, It is very close to absurd for Klipsch to provide for biamping on these speakers. It must be for someone who is in love with a very low powered tube amp, i.e. 2, 3 wpc, 8 wpc.

18 wpc is actually enough for those speakers, up to a certain volume level. If you wanted more volume, you should have larger speakers.

More bass: Another option is to go first to a self-powered sub-woofer, then back to your amp, let your tube amp and main speakers deal only with upper bass, mids, highs.