Speaker Power Requirements

I've heard them driven to great effect by a Van Alstine Synergy 450 amp which puts out about 450w @ 4 ohms .
Bass was very clear and tuneful which is not always the case with these.

I don't think I've seen a bigger change in a companies overall sound like MF when they went from the UK to Taiwan. You would never guess that you were listening to a MF product.

That's bad news, Zd. MF is nice stuff.

I have some experience with the newer MF gear. The 6si will easily drive the OP's speaker. The only problem is that ever since they started making things in Taiwan, the SQ sucks. I haven't heard the newest si version yet, so maybe its improved. Definitely listen to it first.

60 x 30 . Medium sized room? Do you live in a castle - or an old warehouse conversion?

The PSB Synchrony One requires a high current amp to bring out the dynamics which the PS Audio Trio C-100 is not capable of. I have auditioned the Synch One in my home with a 300 wpc/8 ohm, 600 wpc/4 ohm with good results.

You need an amp that doubles down it's power when rated at 8 ohms to 4 ohms, (or close to double). This is because the Sync One's impedance curve dips to 3 or 4 ohms at 1 kHz and near 4 ohms at other frequencies. Without enough power, you will not bring out the full dynamics or detail in the bass.

The M6si looks like enough power, although I'm weary of an amp that only specs it's power rating into 8 ohms. You should call Musical Fidelity and ask if it can drive a 3 to 4 ohm load easily. Otherwise, MF makes very nice gear.