Good question, gonzo. And it's one I've asked myself many times.
The Summits offered outstanding bass response. Man, did those babies punch it out. But I was always fiddling with the 50 Hz and 25 Hz bass controls, trying to get the match right in my room. On some music, which suited the bass settings, I thought there could be no better speaker in the world. Period.
But when playing other music that didn't suit the bass control settings, the Summits sounded overblown in the bass, and there appeared to be some discontinuity between the panels and the bass modules. This was fixed by more fiddling with the bass controls. This was the only reservation I had about the Summits. Otherwise, the Summits were superb.
I think the main problem with the bass response, was that my room is of double brick construction about 6 metres x 5.5 metres and it had a cement ceiling. I believe I had some serious bass node issues which the Summits didn't like. I had a few minor issues with the orientation of the panels - when I had the panels orientated the way I liked them, the bass integration suffered, and vice versa. But I worked out a compromise in the end.
The Montis speakers on the other hand, were an absolute dream to tune into my room. I simply placed them where my Summits used to sit, I set the bass control to -3 dB and I played music. I found the bass was a tiny bit rich, so I reset the bass control to -4 dB, and viola! Perfect integration in my room.
The Montis sounds like a completely different speaker in my room compared to the Summits. They seem to have seamless integration between the panels and bass modules. There is no discontinuity. The bass response is superb. A little bit lean, but with outstanding extension. I think that's where the Martin Logan magic manifests itself. ML haven't tried to impress people with how 'good' the bass is. They've actually taken the time to tune the bass module so that it sounds just like the panels.
The Montis sounds so neutral compared to every other speaker I have ever heard, I have wondered if they would be suitable as studio monitors. Compared to my Montis, my neighbour's Dali Helicon 800 speakers sounds harsh and brittle. My other neighbour's B&W 802 speakers sound more neutral than the Dali's, but compared with the Montis, they lack 'air' and seem more bloated in the bass. Considering I paid way less for the Montis, I am very pleased with my purchase. Just for info, all my neighbours own Naquadria power amplifiers - so there is some consistency in my listening impressions.
It's interesting, but the neighbour with the Dali's suffers minor hearing loss in one ear. The neighbour with the B&W's likes more contemporary rock music than I do. So maybe music taste and medical issues can have profound effect on speaker purchase.
BTW, the Montis goes LOUD! Driven by my 50 watt per channel monobloc tube amps, the Montis speakers can shake the room.
Just some other thoughts... The Montis is cheaper than the Summit X (I haven't heard the Summit X in my home); The Montis is easy to move around; I love the look of the zebra wood option; You don't need t worry about bi-wiring; You don't need to worry about buying a high powered amplifier - 50 watts is fine as long as it's matched by high current capabilities.
Did I say the Montis can go LOUD!