Okay I Bought the Magnepan 3.6's

I don't think the 150.5 is enough, especially in your room. There is a reason so many are recommending huge amps like the Wyred for Sound, Bryston 14Bsst, or Pass 350.5. The maggies are not very efficient and will run through the 150.5's 300 watts in no time. Going to an amp like the Wyred for Sound or big Bryston will net you another 4-5 db's of headroom, a significant difference.

I like Pass stuff, but think it is extremely expensive and not worth twice as much as Bryston, with the 350.5 running nearly $12,000 for a new one, vs. the Bryston's still expensive $6,000. The Wyred for sound ST-1000 as the comparative bargain of $2000 gives you 1140 watts per side, a 6 db increase in output capability over the X150.5 amp. It also leaves a lot more cash in your pocked for other things.

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!

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."