The B&C 6PEV13 was disqualified as it has apparently been discontinued. It's now only in the archived products section on their website. If we were to move to an 8" and move the Xover point lower, we'd be in a range to just use the 10" TDM as the off axis response wouldn't be much different.
In reality, the MTM with pair of TD10M's and the ribbon tweeter really does sound substantially better than the 3way as Jason had mentioned. I'm having a very hard time justifying the addition of a midrange to the system here as we just haven't found one that works as well as one of the TD15's, TD12's or TD10's. The PHL was better response wise compared the unmodified 6ND410, but in reality it's very poor yet compared to any of the TD drivers. In the following curve, the yellow is the 18sound unmodified and the red is the PHL:
In the 6nd410 you can clearly see the nasty breakup from 4-5KHz. There is a large dip from 6-7KHz and another spike at 8KHz. Although they can be EQ'd out in the response, they are caused by a resonance that still doesn't go away. A resonance causing a dip is much easier to tolerate than a peak as it will simply lower the output in that region. The PHL is better, but there is a rise from 3k-8k that also is associated with cone/dustcap resonance. It's a much better damped resonance, but a resonance regardless.
Now compare that to a curve of the TD15M.
No peaks, very flat to 4KHz with a narrow dip above that and then quite smooth rolloff. Impedance curve has some tiny bumps but nothing as significant as either of the smaller drivers:
At this point I need to keep looking for a little bit. In the case of the 3way system, running the pair of TD12S's up to 3KHz sounded significantly better than having the 6ND410 on. The off axis response may not be quite as good, but on axis is much better. I'll see what it looks like with the modified 6nd410 tomorrow. It's hard to put in a midrange driver that has 2-3x the inductance of the woofer, no shorting ring, lower Xmax, etc. The TD10M's with NeoPro5i has proven to be extremely good sounding. I'm considering doing some kind of traveling speaker audition where I could ship a pair to a few people to try out in their own homes. Need to work out the details first as there would need to be some kind of deposit that is refunded or applied to driver purchase.
In the meantime I'll get one of the Beyma drivers on the way also.