From Pass X250,5 to 350,5 : what to expect?

Given the impedance curve and sensitivity of the Salons, you have all the power you need.

I have a X250.5 and driving Wilson Sophia 2 and didn't even think about upgrade to X350.5 ,which will not yield any significant sonic benefit IMO. However, new XA.5 class a series may bring you ultimate class A experience even with more difficult speakers since new XA series doubles output when load drops to 4 ohm.

Having gone from 200 to 300 watts with both Levinson and Pass amps, driving fairly inefficient speakers, I can confirm that more power is better. Better dynamics, better headroom, better low-end control. The only sacrifice for these considerable benefits is the one from your wallet. Well worth it though.

You will get better sound. I the case of Pass Labs .5 amplifiers, the X350.5 is regarded the best among the stereo amps. Something magic is present with the X350.5 but it will come at a cost. The sheer mass of the construction is maybe the answer to you tweaking guys out there. The power you are using for your cuddling moments with Elvis or miss Krall is subtle, never exceding double digits. The obvious difference between the amps is power rating but since most playing is only a fraction of the rated power actually used, the answer has to be somewhere hidden from ordinary wisdom.

>Having gone from 200 to 300 watts with both Levinson and Pass amps, driving fairly inefficient speakers, I can confirm that more power is better.<

That might be true for your speakers/amplifier, but in this case there will be no difference.