Tri-amping 3.6R's; what amp for the ribbons?

class 'a'?
Pass xa30.5 clean 30x2 which near triples to almost 100 in a/b, both while nearly doubling into 4 ohms.
I wish I could afford Pass for my panels. I'd do so without even extensive audition and doubt I'd ever look back.

Hello, Thought I'd throw in my 2 cents as I have a somewhat similar type arrangement. I too run dipole radiating panels and am "multi-amping" my speakers but in a slightly different way. Your 3.6's are great speakers (my first 'hi end' speakers were Maggie MG2B's over 30 yrs ago and afterward I was very happy with MG 3A's for many years). Yours are true 3 ways while my current planars are 2 ways (Apogee Duetta Sigs) which have a separate bass panel and a combined mid/tweeter ribbon. Your crossover scene allows true tri-amping which is very cool. I have what might be deemed "semi-tri-amping" wherein I use an external active crossover (an NHT X-2)to split my main signal at 50hz into a Rel sub, the remaining signal (call it 51hz+) feeds into another active crossover (a fully upgraded Apogee Dax 1) which then splits it into dedicated woofer panel tubed monoblocs (Conrad Johnson Premier 12's) and then to a dedicated amp for the combination mid/tweeter ribbon... This is essentially the equivalent of what you are describing you would like to try. Before I decided to run the mid/tweeter amp I had been using the CJ's and the X2 only, and after the sub relieved the bottom 50hz the CJ's did a great job...(this was utilizing the passive crossovers built into the speakers). Pretty simple, pretty direct...minimum complications and a very very nice sound. BUT, apparently like you, I just couldn't get the upgrade itch out of my mind so when this dedicated external crossover became available I thought I'd take the plunge and pursue the world of TRUE bi-amping...
The story is too long from here so I'll cut to the chase: after a LOT of pondering I knew I wanted tubes (that was never a question) but I also knew I wanted maximum flexibility and of course superb sound. Years ago I ran a pair of VTL MB-125 monos (into Silverline Sonata 2's = awesome combo) and since then became very enamored of the EL-34 tube... simply glorious midrange and a top that can extend as beautifully as the best out there, (especially when driving true ribbon tweeters as both you and I are). I've also appreciated the allure of running them in triode for certain types of music as well as some late night listening sessions but only as an option...full pentode or ultralinear is mandatory of course too. (The VTL allowed switching albeit NOT on the fly) and it was a nice feature.
What I eventually sought out for my application was an amp that could not only meet all of my requirements but in addition, was actually tube-TYPE flexible in case I felt the need to match the 6550's or KT-88's which the CJ's are using driving the bass panels. My answer finally became clear...an amp I had heard about 10 years ago at a dealership; it's a Cary, model SA 280 V12...(mine is an "i"model, F1 edition but that's not the point). It will run EL-34's, 6550's, KT-88's, and a few others as well...all at 100wpc ultralinear OR 50wpc triode as well as varying degrees of 1/3, 2/3 partial triode mix, and all capable of switching on the fly....And it sounds absolutely phenomenal doing dedicated midrange / tweeter duty...I think it would be well worth your while looking into this amp for your application. I will not attest to understanding the math of system/amp synergy in your rig, you will have to do your due-diligence to be sure the specs match up to the rest of your equipment but assuming that all can coincide together, I'm quite confident you will not be disappointed with its virtues...These include but are not limited to its spectacular soundstage, both wide and deep...complete lack of grain, tremendous sense of presence, air and natural timbres... a glorious sounding amp...
I really didn't plan on doing a review here (and I have no association with any brands/dealers/business in the audio field)...I'm just another A'gon addicted Lissnr...
Hope this expands your list of considerations by at least one more...but regardless of this particular amp or not, I would still pursue tubes to handle that tweeter of yours...remember: ribbons (especially midranges and tweeters) + tubes are pure listening bliss.
Good luck and happy Lissn'n

Doing some reading and looking at the components as well as the papers that the crossover is made from for the 3.6r, (I have a pair too and am in the process of trying to find the same sound as you Mikeand1, the Maggies have a4th order L-R Low pass at 280 (not 3rd order at 250) and a first order high pass at 200 which explains a lot reading through the Stereophile review of their measurements.  The quality of components is bottom rate.  A good passive crossover misses the greatest advantage of all active crossovers, there is no part of worry about over driving the entire amp into distortion and phase problems are not as serious, but these can reach 4.5 db higher than a single amp by ti-amping or even higher when it is just one channel at one side that is over driving a single amplifier.  The other is that the amplifiers can be very accurate, articulate without having to spend your life savings.

Problems with ribbons.  You must have a high pass filter one them or make sure that no DC can ever enter through the system to your speakers.  The best active crossovers died in the mid to late 1990s (they are gold with the exception of Accuphase (Kenwood/Trio Lab group).

I am at 240Hz and 1700.  The midrange panel is crossed over at 1200 and not 1700 as the impedance would drop below 2 (tweeter is 3.3 ohms and 4.0-4.2 for the midrange at 1700).  If you wanted a two way one would have to calculate a 4th order otherwise, the dip at at 1300-1600 is considerable, the ribbon being a 12db/oct high pass Butterworth.

If you want output, you will have to go solid state and that is not so bad with a good TUBE FRONT END.  Don't put a Transistor preamp in front of solid state power amplifiers as it sounds horrible and vice versa.  I've had the quicksilver 100w amps with a Threshold preamp and that was terrible.  Audio Research in front of a Bristol amp is a no go either.

The Maggie ribbon is surprisingly inefficient but at 1700Hz and above into 4 ohms (add a Dueland while you can of their best resistors of 1 ohm as everyone is cutting budgets and holding companies are buying out the best of what is the rest of reasonably priced audio (B&W really suffered component wise going to China but it is the bean counters that created their new and rotten crossovers).

I am still in search of a good amplifier that doesn't cost 3000 for a class a of 30-50w/ch.  Some retro amps are looking real good here (Marantz 7 class A, 24 or Pioneer Series 20 30watt/ch and the latter gets VERY HOT and needs a fan but heats the house in the winter.

Spectral will do nothing to refurbish anything of their products and have little idea of the nanoseconds it takes for over several instruments to start on que in classical music or improvisational jazz for just a few.

A solid 100 watt amp like Benchmark (200 w/ch in 4 ohms) is ideal for the midrange and the low end is your own taste but need.  This is where if you want slam, solid state and no passive crossover is needed so all the best the amplifiers can put out is directly to the drivers.