Stacking subs


For years I’ve used a pair of Velodyne HGS-15s placed in the front corners. They are acoustically corrected by a bass manager and seem to integrate flawlessly with the KEF Reference 1s. In a recent post, auxinput suggested I replace the HGS-15s with HGS-10s for LF extension of the KEFs and use the HGS-15s with LFE from the Bryston SP3. That seems like a fine suggestion. I wonder about stacking the HGS-10s in the corners atop the HGS-15s, the easiest and neatest solution. Any thoughts or advice would be appreciated.
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Guys, hi
I am way more ignorant about subs than anyone here. BUT want to ask a related question. My speakers are low on bass but do (technically) reach down to 40Hz. Would I get better bass and a richer sound by adding one or two subwoofer? Running everything with a Lumin t2 to Hegel H390. Everything rocks except the bass...
Thanks.
Yes, especially adding two.  Some argue that high-passing your mains to relieve them of low frequencies handled by the subs duty will enhance their sound.
Do you XO the Kef’s bottom end with any bass management, like hipass in a receiver, if you do XO to the subs at what frequency 
I use a Velodyne SMS-1 bass manager that provides acoustic room correction, and low pass at 80 Hz with a 24 dB/octave slope. I’ve tried high-pass, but it seems to remove a sense of air. That may be peculiar to the Ayre zero feedback design of the preamp and amp between which the filter has been inserted.