Subwoofers - 1 Martin Logan Depth or 2 Grottos?

A previous call for help narrowed my search for a musical 2-channel quality subwoofer - thanks all. First choice was Vandersteen 2WQ but my stupid (albeit sweet sounding) integrated amp does not have power inputs. So forget Vandy. Second choice is Martin Logan Depth. But I was wondering if two Grottos would be better than one Depth? Depth has BalanceForce technology for tightest bass. Grottos do not but two may offer better stereo imaging. Not sure which evil to neutralize.

Current system:
Amp - Opera Audio Consonance a120 Hybrid Integrated (90 wpc)
CD Player - Opera Audio Reference 2.2
Speakers - EgglestonWorks Fontaines
Hi, I didn't understand what you meant by "integrated amp does not have power inputs. So forget Vandy." Also, how big is your room?

Anyway, I have a Depth (one). I was going to get two, but one does the job quite well. It's right between my main speakers and (I guess) because of its omnidirectional radiation pattern, doesn't seem to excite room nodes. I have the phase angle set to 90 degrees as recommended by ML and it mates seamlessly with my CLS electrostats, crossing over at 35Hz. It has extremely fast transient response, and plenty of output for a 12x24x10 room.
My amplifier has pre-outs, but no inputs to the power amp so I cannot place the Vandy high-pass filter between the two to run to my main speakers.
Anyway, I cannot centrally locate the subwoofer except for behind the listening area. My listening area is sort of enclosed as 23'x17' with 11' cathedral ceiling, although it is very open on one side and partially open on the other, with a free-standing 8' wall at the 17' mark in front of the system and more space beyond that (behind the listening area).
My main concern is: Should I address reducing subwoofer cabinet vibration (eliminated by the more expensive Depth) or have two for stero imaging (if I get two less expensive Grottos). Depth has three 8" drivers; Grotto has one 10".
OK, I understand, but I don't think it should be a problem, at least for the way I think a subwoofer should be used.

I would just leave the Eggleston's hooked up as they are (no high pass) and (I assume the Vandy has line level inputs like the Depth) simply connect the preamp outs to the sub(s) line level inputs.

You don't have to put the (single) sub where I did if you can't, but I do feel that the Depth design is probably the least critical of location, the Vandy somewhat less, but it's still down-firing (semi-omnidirectional) and will do OK if it's on a hard surface. I don't think the Grottos are really any different or better than any other single driver mini-sub.
The 2WQ, as you have already stated, will not work with your current set-up.

THe Grotto (while very good) is not really in the same league as the Depth. What ML calls Balanced Force Tech is really just a way of cancelling out cabinet resonances. It works.

If your room and set-up is not symetrical you would benefit little from stereo subwoofers. You may find that two subs do sound good but not as STEREO subs.

Room placement is important so demo it with a 10 meter interconnect to see if the couple of livable places it can go will allow it to sound good. The Depth is the most tweakable in the ML line (more so than the Descent even) and small enough to be less obtrusive than the average sub.

A sub that is not integrated well with the speakers is just a bass box banging away in the corner (no mater how $$$ it is).

Good luck!
I agree with the Nerd. Also, someone here on AgoN had a really nifty way of locating a good spot for (I guess one) sub:

Put the sub at your listening position and play some music with a range of bass, and walk around the room to find a spot(s) where the bass response is flat. That (those) is/are the best place(s) for your sub.
I would suggest that you forget the Depth and go with the Descent, which at a K more will solve your bass needs much better than the Depth. I have tried both, and I much preferred the Descent.
Happy Listening,