The response for REL regarding subwoofers

Great explanation!

You could probably accomplish what you wanted with a pair of No.25 subs from REL, depending on room size and how loud you like to playback your music. 

That makes sense if you’re using the speaker level input. There might be other options, though you’d need to be careful and run the numbers.

With the speakers being so efficient, depending on your listening levels, the speaker level voltage could be within the limits of the line level inputs of the sub. There might be a way to modify the sub’s high level inputs for less attenuation as well.

You could run a sub out from your preamp at line level, but attenuate the signal going into the power amp and to the speakers. This depends on your amp input impedance. A passive pre amp would give additional adjustment. I think some attenuators can have roll off if the output impedance of the pre is too low. 

I’m sure there are variations of these, but it would depend on your specific setup and equipment. 

Horn loaded subs at 95+% sensitivity. Maybe a pair of 12" per enclosure, two enclosures. They are huge but that's what I use to run with Western Electric, VOTT or any Klipsch. Back then we didn't have to many box subs. There ones like REL but passive, the passive radiator was on the bottom too. It wasn't in an enclosed slot. 60-70s not too many manufactures made subs. You had to make them. Jensen Imperials Subs are a ton of fun. With 60-100 watt.. Your bumpin' pretty good.

https://www.decware.com/ImperialSO.htm

I had one and my buddy had one. When we had a party there was one at either end of the packing shed and a pair of Jensen Imperials. 

2 20 watt MACs and 30 watt Mac Kit for my sub and speakers and 60 watt MC60 for his. Would fill the whole joint right up.. 1969-72

That's what you need OP a big fat one dead center of those KHorns :-)

Regards

In 2001, I was using a Velodyne FSR18 with my Khorns and had no issues, mind you the neighbors did....