Anyone use a sub with Reference 3A DaCapo i

No , I have not . I am not a fan of sub integration .

But...
Have you tried rolling the input tubes ?
How far away is your listening position ?
Are you firing the speakers down the long or short walls ?
I have these same speakers and integrated and can get bass
that I can feel , if it is in the music . I am not familiar with your source's sound though .

Admittedly I am in a nearfield situation in a small room but you really can't exspect a small standmount to fill a large room like that .

Good luck .

Call Upscale Audio , tell them what you want to accomplish and try it .

I did and asked about warming up the entire sound while retaining the great topend extension and overall detail .
They recommended rolling the input tubes . I purchased their recommendation from them . I got what I asked for plus some extension in the bass that was also tighter .

You might also try putting the speakers closer to the wall behind them . Would it be possible to turn the setup so that the speakers are firing down the short wall ? If not , could you make the room behind you smaller by hanging a large quilt , blanket etc. behind your listening position . Just get a 10 ft. length of pipe , hang it near the ceiling with closet rod hangers mounted on the side walls and hang a quilt using curtain hangers with the spring clips . Then you can just push it to one side when you want the open space .

I think that it is more important to have the tweeter at ear level than the woofer . Are the speakers firing straight ahead , as Ref. 3a suggests ? Have you tried a little toe in ?

All of these movements cost little to nothing and might work . If not the tubes would be cheaper than a sub which I would do as a last resort as the sub will probably be the most exspensive move .

Good luck .