Spatial M3 Sapphire or Klipsch Forte 4

this nice little calculator will allow you to get a ballpark idea of the volume you can achieve with various wattage.

https://myhometheater.homestead.com/splcalculator.html

I would think that your single ended tube amp will give you a really nice midrange, top end and soundstage but not the best control of the woofers for the really deep stuff...but if you are committed to staying with the subs...then it might not matter.

There are a lot of threads on AC regarding the spatials and how they mate with low power, high power, solid state and tubes....and how they compare to other speakers...they are worth reading...

Here are a few  links..... 

https://www.audiocircle.com/index.php?topic=180075.0 

https://www.audiocircle.com/index.php?topic=180361.0

https://www.audiocircle.com/index.php?topic=180031.0

I own the M3 Sapphires driven by a PrimaLuna Dialogue Four. This is a push-pull amp that can be used in either ultralinear (42W) or triode (21W) mode. I prefer triode mode. I listen really loud at times to a wide variety of music. Very open, realistic, and dynamic sound. The soundstage is very much behind the speakers, so if you prefer a more intimate presentation (i.e. performers in the room with you) this may not be to your liking. The Sapphires are more like sitting in a large auditorium watching the performers. Bass is tight, accurate, and detailed, but to some may ultimately lack sheer slam and heft. It’s not that the woofers are insufficient–there’s four 15" paper cone woofers that can move a lot of air. Rather it’s the open baffle design and lack of cabinet reinforcement and resonance.

Klipsch’s are also very dynamic and usually extremely efficient. Low power SET amps can be used. I’ve only heard Klipschorns, but I would say if you mainly listen to classic rock and want to listen loud then the Forte IV’s may be more satisfying. The Sapphires will sound completely alien at first, but once you acclimate to them you will appreciate what they bring to the table.

This is all to say audition them both if you can. Since you owned the Forte III’s you have a good idea of what the Forte IV’s will offer. The Sapphires will be different.

@ozzy62 thanks. I need to find a local dealer to go listen to the CW IVs soon. 

If I had the room for Cornwalls I probably would have bought them, but I bought the Forte IV and have ZERO regrets.    I love these speakers .    They are so good I revamped my amp and preamp after buying them .   I'm actually having a 300b built.  I almost bought the Forte III after having owned Hersey III but the new mid driver is much better so I waited for the IV.

It’s mostly the amp. I too have OBs, in my case Emerald Physics 3.4s (concentric 1" polyester in a 12" woofer) and my room is ~ 35 x 20 x 12’ peak in an open beam ceiling. Front wall is ~ 80% glass and rear wall is 100% glass. A tough room to get thumped in. I tried using 2 SVS barrel powered subs, which I bought new 10+ years ago. In those days they were focused on HT, and these sucked for 2 channel. I sold them about 2 months ago, and my musical enjoyment is the better for it, but, no deep bass, and missing a bit of the muscular musical scale. By far the 2 biggest changes I made were replacing my EVS 1200 class D 600/1200 wpc dual mono amp with a LSA Voyager 350 GaN amp. And although the V has ~ half the EVS’ power it is actually a lot more room filling. The other addition, which significantly improved low end (as well as the rest of the spectrum), was replacing a Marantz HD CD-1 that I used as a transport with a dedicated transport- Audiolab CDT 6000.

I still want what good subs can bring, but the cost to do it right is elusive to my retired SS budget. HTH