A lot comes down the efficiency of the speakers. The Synchrony One's are world class speakers, the top of PSB's line. They are also 4 Ohm speakers with sensitivity of 90 db making them more difficult to drive. This is probabaly why they were mated with the McIntosh. The Paradigms are far more efficent - 8 Ohms and 93 db sensitivity. They need less watts to acheive the same volume level as the PSB's. But this doesn't mean you need 250 watts for the Synchronys - they should perform very good at far less power. Provided its clean, high current power and the amp is not overdriven into clipping.
Were both speakers you heard at the same dealer? If so, why don't you re-listen to the Synchronys with the 50 watt amp. Providing the the amp is 4 Ohm stable and will not clip, this should be adequate to get the PSB's up there in an averaging listening room. If you still think the PSB sounds better, then you know it is this speaker you have a preference for. There are many higher end 4 ohm speakers being driven by 50 watt amps in folks homes all over the world. My speakers, Vienna Bach Grands are rated the same as the Synchrony's - 4 Ohm and 90 db. I drive them with 125 watts per channel and never have to push the volume control to achieve satisfying volume. This is only half the power of the McIntosh you auditioned. Good luck in your speaker search.
Marantz PM15S2 LE, Oppo BDP-103, Simaudio Moon 100D DAC
VPI Traveler V2 with an Ortofon Rondo Blue and Simaudio Moon 110LP, Technics SL-1400 with an Audio Technica AT-14sa and Pro-ject Phono Box S
Dynaudio Focus 260's, Rel T-1 and a Panasonic TC-P54S1.