Thanks JLM,
Maybe I should provide some details of the current speaker setup. In the top I'm running TPL-150H from 7000 Hz, first order. Mids are Radian 950 16 Ohms from 800-7000 Hz, first order for both. They play into JMLC 270. Upper bass/low mid are Faital M5N8 12 Ohms from 150-800 Hz, first order, playing into 140 Hz tractrix. Bass are (currently) vintage Altec 416a to 150 Hz, second order, in some super sturdy 200L-ish reflex cabs. I've also been using 18" BMS 18N850v1 for bass duty and haven't settled yet, depends partly on where the'll be crossed over. My room allows for experimenting so down the line I may fiddle with anything from TH, IB, OB, corner loaded horn, etc. but prio now is on the other channels.
Despite the smallish room of roughly 26 square meters I won't simplify to a 2-way just yet as I'm having too much fun. The closest I've got is trying BMS4592ND coaxes in the 140 Hz horns but despite the point sourciness I didn't think it worked that well. Way too big horn to also cover treble duty in my view, leading to coloration. Some seem to be pursuing this track successfully, but I haven’t heard it myself. I've been trying JBL 4560 clones (horn/BR thingy as you mentioned) with both JBL K130 and the Altecs which didn't cut it during my attempts (actually opening up the backs to run them OB-ish didn’t sound half bad). I could do some more 3-way mix and match explorations with the stuff I have at hand but that would likely imply skipping the 140 Hz horns which would be sad as they are too interesting in this range, although it does get a bit more forward. Then again, most other non-horn stuff playing this range have a hard time keeping pace with mid horns.
About the size of bass horns, a friend of mine have a bespoke all front horns 4-way system in concrete in a moderately sized living room where it plays flat to 16 Hz and the music is totally decoupled from the speakers and the crossings are seamless. His bass horns take up a nice portion of his basement. Another friend have a bespoke 3-way all front horn garden system to get rid of all wall/ceiling reflections where the speakers are 6 meters apart. A huge mono bass horn sits in the middle which still doesn’t go under 30 Hz on paper but plays at least as vast as an outdoor PA except it’s got high end sound instead of the usual mush. He also has a very nice bespoke indoor horned 4-way (A7 mid bass and TH for subs) so I got some nice reference points of what big horns are capable of in a few different settings. I have also heard with my own attempts when things gets colored and congested which is mostly when I ask more of the horns than their range allows. As you said, physics. One could argue asking the 140 Hz horns to go down to 150 Hz is just that, despite simulating and measuring ok. In the end everything about recreating recorded events is a compromise to some degree.
I realize some of the stuff I combine isn't 100% optimal and some might wonder why I've changed a lot of pretty competent gear around but parts of it I just happened to have at hand from an earlier rehearsal studio, some I got at very reasonable prices as combo deals when buying other stuff and a few boxes where about to get tossed if I hadn’t saved them. It’s still a low budget system.
About hum and hiss, I’ve heard another fellow horn aficionado’s Avant Garde Trios in a 100 square meters listening room with an LM219IA amp that was too hissy for my taste but the other above mentioned systems’ bespoke amps are dead silent. In reviews NuForce/NuPrime are always said to be dead silent even paired with higher sensitivity speakers. Still, silent at e.g. 94 dB may not necessarily equal silent at 111 dB, but maybe it is. Mild on/off thuds are fine too.
Anyway, this thread was originally about finding great sounding and easy to work with amplification to match my stuff so I hope I’m not wasting anyone’s time rambling about horns.
