He's using bass guitarist lingo. Generally 'slam' in the 'mids' would be around 120 to 300 hz, extending a little above the 'punch' of a kick drum you may be thinking of. Without knowing what pre he was using and how the Crown was fed, there's no telling what was going on, but bass amps are rarely flat and often have a baked-in boost in the low mids, so a flat amp would sound light there without EQ (which any bass pre would have.) I'm surprised he didn't complain about 'cutting through the mix' but I have no idea what sounding 'plastic' means.
Bass players have a lot of odd superstitions and one of those is that the heavier the lead-sled amp the deeper the bass, thus light class D amps must clearly have trouble pumping out the low end - an idea slowly dying now that the non-tube bass amp market has completely converted over to class D.
Old habits die hard, but even Stereophile is drooling over the current crop of Class D amps. As to boat anchors I still have a couple of 500W Classe Class A monoblocks that truly dim the room lights when turning on--can almost sense the whoosh of charge flowing into them.
I consider selling them every so often, but there's some nostalgia working against my better judgement. Also a reminder of how foolish I was. I was just driving some Thiel's and later Dunlavy's with them, both dynamic driver speakers of medium efficiency.
Talking about changing times, I grabbed those second hand for 5k/pr. The original price was 5k apiece, which in todays dollar probably 30K. Now a 1000 bucks buys you some serious effing amplification, that is likely every bit as good, and for what 2000 Euro for the 400W x 2 Hypex stereo module possibly better specs.
The Classe's do double down all the way to 2 ohms which is remarkable, so you can imagine the iron in those monsters. But hot as hell and unreliable as well--which is likely why I got them nearly new for 1/2 price. God I was dumb. Now I am looking at Crowns for the same 500w/side and prepared to spend oh maybe 800.
Which is why I am posting. The product line is so diverse, I'm a bit overwhelmed by the choices, and the web site I'm not finding helpful. What I need is about 250 W/side and 3dB of headroom. Wouldn't mind having some DSP, but will be doing that upstream eventually, just that EQ and delay may be handy to have at the amps.