Magnepan 3.7 - 3.7i owners need help please

Sorry to hear you're not ecstatic about your 3.7s. I have owned the 3.6Rs and the 3.7s with the Sanders amp although with a Hovland HP 100 preamp. After break in I found the 3.7s more open and dynamic with a more detailed soundstage. As others have commented break-in is so essential. Keep the faith. 

After you've done your break in, if you are still not happy with the sound, I would first experiment with placement of the speakers and toe in.  Like any fine speaker, placement is everything and they might end up being placed a fair amount different in your room than the older pair.  Put two strips of painters tape on the floor and mark them in inches with a pen and proceed to move the speakers an inch or two (at the most) forward and back until you hear an overall improvement compared to other positions.  Then experiment with toe in. As a dealer I find that even long term audio buyers/audiophiles do NOT really know how to precisely set up their speakers to make them sound their best.  If you are unsure of your competence in this, please call your dealer your bought them from and ask them if they would do it for you.  Most will gladly oblige, often at no cost since you just spent a bundle on the speakers with them.  Of course, if you didn't buy them new from a dealer, then you may have to pay someone to do this, but if the person is knowledgable and practiced at speaker set up, believe me, it will be worth paying for a couple of hours or so of their time. Good luck!

Please examine the book Get Better Sound by Jim Smith, who worked for Magnepan for many years, who has provided some further guidelines for speaker placement that may lead to large sonic breakthroughs for you.  Book is very modest in cost, can call 1-770-777-2095 to purchase direct from Quarter Notes Press

I  spent many years with MGIIIs then upgraded to the 3.7s (not i) and they took several hundred hours to catch up to the IIIs.  Your amp is excellent, I use the Magtech with my 20.7s and would NEVER switch. Read the Magtech reviews on Rogers website, one of them was done at my house with my 20.7s.  One suggestion that I will +1 on is to get Mye stands, the 3.7s really benefit from the additional stiffness from the Mye stands.  Another tweek that I found is to use a tube pre-amp in front of the Magtech, I started with a MC 2300 and NOS Mullard 4004s in the driver stage, and now use the MC 1100 with the same Mullards, a truely perfect combination!

I have a pair of 30 year old Maggie IIIa's and a pair of new 3.7I's and have to agree that the newer Maggies took hundreds (300-500) of hours to break in. Until they broke in, the soundstage was diffuse, and the bass was less than stellar.  Honestly they were a mess until breaking in. 

The new speakers definitely added a strip of felt behind part of the midrange. My guess is that it is to dampen the back wave reflection and allow the speaker to be placed closer to the FW, thus making the speaker more spouse friendly. It is obvious when looking at a 3.7 vs a 3.7i. Magnepan is famously reluctant to give details though. 

One tip with the i series is that they need -- according to Wendell --  to have the woofer panel slightly closer than the ribbon tweeter. This of course means that if your tweets are in that you need a lot of toe in, possibly more than you are used to. Less toe in with tweets out. You can see the effect of getting the tweets further away in measurements as it mostly reduces a suck out around the crossover which is readily apparent if the tweets are the same or closer.  

I have not heard the 3.6, but compared to the IIIa, the 3.7i has more detail, has a much smoother and tamer upper midrange, more upper bass and substantially less low bass.  The older speakers go down to the upper twenties while the new ones end in the upper thirties.  (In the 30 years of evolution they made the woofer panels smaller and the midrange larger).

i am currently running both pairs. The 3.7i's are being run in my smaller room with Pass 250.5 amplification and are about 7.5 feet from the FW, tweets in lots of toe in. I am running the older pair in a much larger room tweets out, about the same distance from the FW, with DWMs to boost the mid bass.  I drive the second pair with the 1700 watts into 2 ohm Emotiva XPA1s (the DWMs and larger room make the tough Maggie load tougher).

I prefer the Pass Labs and 3.7i, but there are pros and cons to both systems.