New Klipsch Forte III’s

When I first got my Forte IIIs, I had them connected to a Vincent SV-500 integrated amp.  Had them placed very close to the wall, somewhat close to corners.  No subwoofer.  They were sending earthquake-like bass all over the house -- it was incredible!  I've changed my setup a lot since then, including going to much smaller speakers and a different amp, but that bass was memorable!  When I switched to tube amp, the bass pretty much went away.  I'd say blame the room, blame the amp if you're not getting good bass.  No experience with the A21 myself, though.

@redwoodaudio  Interesting. I have to say, I'm using a tube amp with my Forte III's and I can't completely rule out that the less-than-optimal bass is due to the tubes/transformers which may be challenged. In any event, the HSU completely filled n what I was missing and the mids/highs are glorious.

Last_lemming,
I demoed a pair and sounded as you describe. I don’t know how much time was on them. I would add subs but I add subs to everything lol. 

I have Forte IIIs and have what I consider to be good bass. They're being driven by a Rogers tube amp. I also have a Hsu powered sub, but leave it off most of the time.

Since they have the passive radiator on the rear, they are sensitive to placement. Try moving them in your room (both toward and away from the walls) and see how that impacts bass. 

I bought mine used so I can't provide any insight into break-in. 

If you have the REL why wouldn't you use it? I think if properly integrated it will give you what you need.