Sonus Faber Stradivari amp: X600.5 or X1000.5 ?

Check the specs of the Pass amps. I think the X350.5 has about the same current as the XA100.5. The XA100.5 also has more class A watts and doubles output as impedance is halved, so 200 watts at 4 ohms. I would stretch for the biggest XA.5 you can afford, BUT your Strads are pretty efficient, though I don't know the impedance, so the XA100.5 should be plenty. Of course, you should listen to be sure.

Johnb30:

Even at normal listening levels, peak signals can pull far in excess of 80 watts, which gives you the crossover distortion problems of AB operation. Full Class A operation has a lot of practical disadvantages (very high electricity consumption, a lot of heat, need to periodically re-bias output transistors), but manufacturers still make Class A amps, despite the practical disadvantages, because sonically, full Class A operation, when properly implemented and all other things being equal, sounds better.

In addition, all of the Pass amps, including the 160, have dozens of output transistors - on a transparent, finesse speaker like the Strads, especially given its very high sensitivity, you're generally going to get better sound using a high quality amp of 100-150 watts/channel that has a very small number of output transistors. The darTZeel, for example, is a zero feedback design that puts out approximately 150 watts/channel with only one pair of transistors per channel. The Pass is a very fine amp that has few gain stages and little or no global feedback, but you're going to hear the effects of all of those transistors in a high-resolution system.

Raquel:

Stards is haveing 4ohm nominal impedance so this 80w of pure class A will double ? Then 160 watts should be enough for "normal" listening levels.

Johnb30:

That's not the way it works. Again, every time there is a fortissimo passage, the speakers can draw far in excess of 80 or 160 watts, and the amp operates in Class AB. For pop or orchestral played at "normal" volumes, this happens all the time, which means that crossover distortion is always an issue. If you are listening to chamber music at 75 decibels at the listening chair, this won't happen, but it happens all the time with pop / rock / orchestral / big band jazz, etc. Again, if it wasn't an issue, amp manufacturers would not bother to make full Class A-biased amps, which are not easy to own (enormous heat and tons of electricity - mine draws 1,400 watts at all times and gets HOT).

I suggest that you make a judgment call as to just how much loud music you listen to. John Atkinson estimated the Strad's sensitivity at 93 db., which means that it is highly efficient - you do not need a high wattage amp to make such speakers go loud, just a well-built amp (i.e., stiff power supplies) that can deliver current (it drops to 3.3 Ohms in the bass). If you intend to listen to a lot of loud music, however, why not buy a loud music speaker, for example, big Wilsons? In any event, unless you do most of your listening at really loud volume, a high-quality 150 watt/channel amp, especially one that does not use global feedback, should make you happier in the long run.

Don't get so hung up on "watt" ratings. I've read where XA60.5s were compared to X250.5s...and were preferred.

How can that be? A lowly "60 WPC" class A amp slugging it out with a "250 WPC" class A/B and coming out on top?

It's telling that XA100.5s can be had for about the same money as the X600.5s.

The XA.5 amps are just in a different league.

And I own X260.5 monos and XA60.5 monos. I run the XA60.5s daily. I really like the X260.5s, but they don't outpace the XA60.5s, and I like the sound of the XA.5s better.

Think on that: I'm not sure how much "Class A" watts the X260.5s are supposed to deliver, but the "little" XA60.5s are more than a match, even at high volume levels.

If I had the funds, I'd have all XA.5s. Whatever the class A rating of the X.5, I wouldn't care. A "smaller" rated XA.5 will more than compensate. I'd really like to compare a pair of X600.5s and a pair of XA160.5s...I know where my money would be.

Also, the XA and XA.5 amps are different machines. Try not to get highjacked by any comments about XA amps and their relative performance.