Monitor enthusiasts: musical subwoofers?

Stereo subs, ideally REL..unless you own a TACT preamp, in which case the TACT subs work better. I own a pair of REL's now, and waiting for my TACT's to arrive. My monitors are the SF Guarneri

Can't comment on the Stereo/Mono question, but REL is certainly the most musical, and coherent sub in my experience with 5 different brands. Others might impress for a short listen, but over time the Rel sounds more natural when dialed in. I like smaller drivers because I like to hear more pronounced plucking of the strings over maximum bass energy, but that's a personal taste thing.

Pair of Vandersteens used or a pair of ACI Forces.

My nearfield setup consists of Modwright Swans M1 mini-monitors (4" Kevlar/paper woofers) and two NHT SW-2Si 10" ported subs. No money for REL. I cross them over 24 db/oct @ 80 Hz with a Marchand X9 Deluxe electronic crossover (modded by Dan Wright). My objectives were musicality, seamless transition and invisibility of the subs. Right on.

My plan is to get the Decware horn loaded sub. I heard Kevin Barrett's DIY horn loaded beast and a pair of VMPS Larger subs with a Marchand X9 and there is no comparison when it comes to speed and musicality. Horn subs win.

you should get 3 identical subs for the front array, but for best effect don't plug any of them in!