Which Would You Choose?

Hegel New H 600 is far better then both.

Tube amps too weak on the bass drivers the Hegel has a class leading

4000 damping factor which is far better then. The competition by a large margin,

giving far better control and slam to all drivers.  And the Hegel H600 

has a excellent preamp section to complement the Dual mono amplifier section 

and a very good dac section even better streamer that has tidal but will have Roon to add very soon with QObuz and other software upgrades. Substantially better then any previous product far closer to their best separates. I am considering this for myself.

To answer the question you posed, I own the Michi S5 and I have been very satisfied with the performance.  As someone said, it's probably not going to compete with an amp three times the price, but that should be no surprise. I recently was looking to upgrade, and auditioned against Macintosh amps and preferred the Michi over the Macs and Classe. I also listened to Boulder as well, which certainly was better but of course at  about three times the price. In my case, i thought I could do better by keeping the michi and spending the available money elsewhere. I find the Michi gives you abundant power, and pairs well with SF. So, I think the S5 offers a lot of performance for the money.
 

I have not heard the MoFi. I don't have the ability to review components, I have not heard, at least with any hope of offering a helpful opinion. As with most buying decisions, I think it comes down to getting the best performance for the money you have to spend. Obviously, if money is no object, upgrade everything. 

Be careful what you ask for on this site. OP is thinking of switching amp and some recommendations given are you need to upgrade EVERYTHING LOL.

Thanks all for your input. 

If I change my preamp my choice would be a Bryston BR 20.