40Watts Stereo ISP STEALTH power ?

I have a non-powered "toaster" Kemper. For the last two years I've been gigging with a Mackie Thump 112, 1300 Class D watts. Like most of you, I find it sounds good but the Thump has an abundance of boomy bass and the high end can sound brittle. I'm mulling over powering the Kemper with an ISP Steath. My cabinet of choice is the new Yamaha THRC 212 300 Watt Speaker cabinet. It's got two different Emminence 12" speakers in it that handle 300 watts @16ohms mono, and 150 watts stereo at 8ohms. The ISP has two impedances, 180 watts @ 8 ohms and 90 stereo at 4 ohms. Running the ISP into the two 8 ohm inputs will yield 40 watts a side. The ISP Stealth is an analog power amplifier, not the usual Class D. My question is will 2 channels at 40 analog watts cut it in a Classic Rock Trio. We're not Metal but we kick ass. Is the analog 40 watts comparable to "tube" 40 watts? Can I assume 2 channels at 40 watts will yield close to 80 watts? I need clean headroom in that we do anything from Nowhere Man and Sultans of Swing to Sharp Dressed Man. I'm OK with not having FRFR speakers. The Yamaha cabinet was designed for their THRC 100 watt heads. I'm OK with a little speaker coloration. I want analog warmth. THANX