Intersting question about two of the best <$1000 speakers we have seen in the last 10 years. I have owned both with the intent of accomplishing similar to what you describe. As a side note- forget about horizontally biamping MMGs with two different amps- the xo point is too high for this to be done with good integration IMO. Ok here is my take;
The MMG has more potential overall. The MMG will throw a wider, deeper and taller soundstage. Treble/HF is substantially better with the MMG vs the Titus. If you had a dedicated room and were looking for the absolute best sound it would be my reccomendation especially considering your B&K amp has enough power to wake them up. However, for many reasons I am going to suggest going with the Titus.
The Titus is slightly more coherant overall and also edges the MMG in midrange naturalness- especially when tube driven. It is faster and livlier than the MMG with quicker attack and transient response. (A quick guitar riff will excite your room nicely with these) The 202 version can be powered by just about anything, the ES only needs 8-10 watts. I would stay away from the XS- it is a nice speaker but bettered by the newer versions.
Now- here are the more complelling reasons to go with the Titus. The Titus is easy to set up- it can be placed close to walls and has a larger sweet spot. Most importantly one of its greatest attributes is that it comes alive at very low volumes. Low volume listening is delightful.
The MMG is exactly the opposite in all regards. It is a pain to place, needs to be far from walls, has a small sweet spot and needs to be played fairly loudly before it really starts sounding good.
Im not sure what your office environment is like- but typically the Titus would make a lot more sense.
The next question is which Titus? The 202 is better at what these speakers do best. Midrange midrange midrange. On first listen it has a remarkable amount of treble which takes some getting used to- i.e. break in. In most rooms however this is not a problem. The ES has a larger cabinet, and is really more like the Comete from the 202 line. Overall it is a better performer with a more refined top end. The xo is a little bit more complex in the ES which prohibits most SETs. Your GTA will be more than adequate however. The newer ES Titus is still fast- has a better bottom end and is better looking imo. I have owned both- and think the 202 is probably better only if you go very low power tubes. With 40wpc being static- the ES is probably the better choice.
In the end- the MMG and Titus are very different. Listen to each if you can- but at face value I give the Titus the nod.