It's like this I feel. Having held the massive cone/dome/coil assembly of the 20 drive unit (is the 19 a non-SL version?) i n my hands, I can tell you it's a rather heavy thing compared to conventional cone-coil assemblies. the short coil-long gap arrangement also loses a hell of a lot of efficiency too. To properly support this heavy assembly, the suspension needs to be VERY taut to prevent sagging (my 20ASL pros did suffer a sagging cone after several years and so I inverted the driver in an attempt to 'fix' it).
The 20's can be used by a traditional non-foo 100WPC amp, but if you like some headroom and overall level as I once did, said amp WILL clip. I used a freshly updated Quad 405-2 sat between the speakers with longer interconnects as recommended by ATC and it used to get stressed as well as bloody hot when driven in 'spirited' fashion into these (almost too hot to touch) - sounded great though... I replaced it with an AVI S2000 80WPC stereo amp which sounded the same and ran stone cold most of the time, barely getting tepid if pushed for long periods. Next were the 140W AVI S2000 mono's which went louder and again, absolutely no stress at all. later, I was able to use SCM20's with the AVI 200+WPC S2000 model (the 17" or so deep one) and that was effortless.
Fast forward to today and I really wonder if the Neumann KH150 wouldn't do it all a lot better? They're three grand the pair though and have amps and so on built in, but used 20's are usually half that and you need to buy an amp for them (possibly a European sourced Hypex or Purifi based power amp?).
I doubt many of any here have done it, but I seriously wonder if a pair of 11's or pro 12's wouldn't now be a better balanced and easier to drive speaker than the 19 or 20SL passives which are over thirty three years old basic designs? I heard the 11's driven by a Naim streamer (so rather low powered in today's terms) and they were as happy as anything and had some bass too
The driver seems more 'normal' in configuration..
If your room is large though, as said above, you're not going to get much out of any small box no matter how hard you work it! The rules of a good big speaker being better than any small one still apply, even in these days of active drive and DSP and so on (the distortion below 80Hz is always HUGE on small hard working drivers and this must affect the midrange too if a sub isn't used - why use a sub when a bigger speaker might integrate it all better?). GOOD big speakers cost money though and I apologise for that, but you don't get owt for nowt - and that's something I've learned the hard way in life