New Klipsch Forte III’s

I have a pair of Forte III's in a 2nd system and I agree, the Forte III's don't produce enough bass as they only go down to 38Hz.  I added a HSU ULS-15MK2 sub and the sound is perfect, filling in that low foundation and improving every aspect of the entire range. I don't think you'll get that much improvement from break-in alone.

You could bi-amp from your BAT pre using the RCAs to the parasound and the XLR to a nice VK-220/225 amp. Not only would you have more power overall but the output via XLR is like 3db greater giving you a nice bump to the bass! I briefly heard the Forte III in a showroom setting and it had very nice low-end output. 

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.

Buy a second REL.

certainly not known for huge bass but the bass should really blossom in a few months. let it ride and report back 

Have the Forte 3 as well. I found the best bass response when positioned closer to corners and a foot away from the walls behind and beside. Also I personally find the Forte 3 performs better using SS amps vs Tubes. The extra power gets the cones to really move and do the job these big woofers were intended to. More bass to ya! 👍

Thanks for all the responses. I will ride it out a few months and check back. I have nothing specific against using the REL other than I like the idea of all the sound coming from one source. 
I will tell you I am loving the big sound these speakers give. 
Though I’m not a classical music guy, it gives you the feeling you are definitely there if you know what I mean.