Okay I Bought the Magnepan 3.6's

Mcreyn,
what's your basis for writing that the X150.5 will not be sufficient power for the Maggie 3.6/Rs??

I took a look at the 3.6/R impedance curve in the S'phile review. Here is the link:
http://www.integracoustics.com/MUG/MUG/reviews/stereophile_mg36r.htm

scroll down to see the imp & phase plots.
What I see from these plots is that the speaker impedance & phase curves are quite well-behaved & that there isn't much phase shift across the audio band. We are talking +/- 35 degrees max w/ the phase being at 2.5KHz & higher. In the bass frequencies I see that the speaker is a relatively constant 5 Ohm load - very little phase shift in the bass freq.
So, IMO, a Pass X150.5 should be enough. My calc show that you will need about 40W/ch to produce 90dB SPL in your room sitting 10' away. The rest of the wattage will be for dynamics. My calculations show that you have 5dB of headroom for dynamics & I think that's quite good.
I do not know how much current the 150.5 can deliver but current delivery of the amp will have a bearing on how well the amp's wattage increases with decreasing speaker load. If the amp has enough current then you'll get 300W/ch into 4 Ohms & that appears to be sufficient for the 3.6/Rs in your room.
More wattage (along with current to support increase in wattage) is always better but it gets expensive on your wallet!

I recently used a Pass X350.5 with my Maggies, and the sound was great, but it did strain a little at volume, and was not quite on the same level overall with Maggies as the best amps I have used with them: Cary CAD 500 MB's, SandersSound Mono's, Brystons.

I think that next level up on the Pass (X600's) are probably better than any of these, from what I heard on my days with the X350.5's, but I would take any of these over the 350.5's right now.

Thanks for all the very interesting feedback, comments, and recommendations. My final analysis of the Mac 402 is that it is not the amp I want for my rig and my new 3'6's... I'll have to look elsewhere. Still leaning towards one of the Pass or possibly trying the Wyred 4 Sound ST-1000; in any case I am still looking!

If you happen to live in the Atlanta area and have one of these, please let me know.

Bombaywalla,

You are correct the Maggies are a relatively benign load in terms of current requirements; as long as the amp is comfortable into 4 ohm loads, there shouldn't be a problem. What you are forgetting is that Maggies do suffer from dynamic compression that gets worse the harder you push them. Your calculations would be correct if they were linear, but they are probably suffering from 2-6 dbs of compression in the mid 90 db range.

My experience with my 2.7s was that with 200 watts behind them, they were completely underwhelming. When I borrowed a friends older Yamaha M80 amplifier (330w/ch) I flat out would run it out of power, the VU meters would light up like a Christmas tree in Times Square. It was only when I bought an Adcom GFA-5500 (350w/ch with 1.7 db of headroom i.e. 500 a side on peaks) that they started feel better. Even with the Adcom, I would light up the clipping lights on loud passages. I ultimately ended up with a Sunfire Stereo (600 per side) and if I could swing it would go to the signature version at 1200 per side. This was all in a 14 x 17 foot room.

The x.150 puts out 300 watts/ch into a 4 ohm load, which as others have said is not adequate from their experience. My experience says that 300 watts/ch in a smaller room wasn't adequate, so I have no reason to believe it would be in a larger room.

The article you cite in Stereopile sums it pretty well when the review says "Another consideration is that although the 3.6/R is a benign load—mainly resistive and a fairly flat 4 ohms—at 86dB/2.83V/m they’re not terribly sensitive. The VAC Renaissance 70/70 is an unusually strong 70W amp, but wasn’t really enough to make the Maggies sing. The Mark Levinson No.20.6s, VTL Ichibans, and Classé CAM-350s all did better jobs of resolving low-level dynamics and detail, and opened up the soundstage noticeably. I spent time with all three, but ended up preferring and doing most of my listening with the Classé monoblocks, which are rated as delivering 700Wpc into the Maggies’ 4 ohm load."

Don't get pushed by some of the above posts into thinking you must have 350wpc, 600wpc, 1000wpc, 1,200 wpc. And the post above misstakes my experience that the Pass x150.5 was inadequate - wrong!

Of course, if you want to spend the $$, then you can't go wrong with a higher powered amp, other things being equal.
But, as I said above, it depends on how loud you play the maggies. The Pass 150.5, as I said above, is really a 250wpc amp disguised as a 150wpc, as per the Hi Fi News tests and is much more than merely adequate.

My 150.5 has more power than my prior Classe CA-201 (stated as 200wpc). My friend's 150.5 had, in his opinion, more power than his prior Classe CA-401 (400wpc), particularly in producing deeper more powerful bass than either of the Classes, which both of us felt were somewhat on the soft side on the bass. The Pass was better in all other categories also - detail, transparency, etc.

But, if you are unsure about the 150.5 and want to ensure adequate power but not go overboard on the power/price issues, I would say you can't go wrong with the Pass X250.5.