A few thoughts from me.
The price on the S250 seems very good...especially from the dealer. I'd jump on it. I don't think you will beat it as a stereo amp. If you're thinking of stretching you budget, the DAG pre would be a sweet combo....but let me interject another thought...if you end up with the S250, have you considered putting a little more money on speakers instead of the pre? Move up to the Yvette from Sabrina and I think you'd experience a MUCH bigger jump than with a DAC or Pre.
Regarding a DAC...I'm not sure what Mytek you are referring to....but take a look at the Manhattan II for $6K retail. I think you'd be extremely happy with this as DAC and PRE. It is an outstanding piece. Don't for one second think it is in any way slumming it. I use one direct to a Rowland 625 S2 and love it. I recently had two very high end preamps on loan and wasn't convinced I needed to own one yet. I also used the Mytek Manhattan directly to a DAG S200 and it was a superb combo.