The Bryston 10b is no slouch....(I see it on the list now)
The pioneer units, and the nikko, and at least one sony (see those as well) unit can also go on the list, if they are internally upgraded. Marchand is also on the list, if the same is done. but many of these are in need of refurbishing, regardless.
oh yes, the audio research units, but they will also need refurbishment.
I use the low pass section of the Dahlquist crossover, which uses active devices to provide an adjustable 18db per octave slope. Mated with the active internal high pass filter on my Beveridge 2SW amplifier/speakers, the crossover point is "seamless". As a general rule, most of the tube based crossovers, Marchand excepted, provide only a 6db per octave slope, like the ARC one. Solid state crossovers are more likely to provide higher order slopes (12, 18, and/or 24 db per octave), typically. (There are exceptions to every rule.)
Lew, I think Roger Modjeski (who designed the electronics for Beveridge) has an updated/improved x/o for the 2SW, as well as a new dedicated direct-drive tube amp for the speaker.
I had the Dahlquitst DQ-LP1 for years, selling it only after I got a First Watt B4. The B4 is unbelievably versatile: high-pass and low-pass 1st/2nd/3rd/4th-order filters in 25Hz increments from 25Hz to 6375Hz. All discrete (Toshiba Jfets), no opamps or IC’s. Unfortunately, the B4 is no longer available assembled, only as a DIY kit. Nelson’s Pass x/o is even better, but sells for a lot more (the B4 listed for $1500).
I used Threshold 1x and 10x crossovers with Focal Bass Drivers (2-10" Polyglass Drivers: Polyglass technology, exclusive to Focal, consists in applying molten glass microballs on a cellulose pulp cone), with Accuton midranges and tweeters. Pass designed Adcom GFA 535 II, GFA 545 II and GRA 555 II were the amplifiers. I was still using a well broken in Pioneer C-21 (look at the PCB board and the thick traces of very high quality as well as some of the best components one could buy but it took a very long 500hr + breaking time otherwise it was far too bright).
If you want to see the difference between most of the garbage today and true engineering prowess view www.thevintageknob.org/pioneer-C-21.html