edit- whoops, didn't see your 'new' qualifier. following pertains to old model, but may still hold true for new. I will come back and edit once I've heard it.
Warm, but not dulled in the treble. Opamp choice has a lot to do with the final sound, but most will still have at least some warmth to the sound. The LT1364 I use is one of the brighter opamps, but it still is warm compared to other amps I've used. Highs are very opamp dependent. It can be edgy and rough or soft and smooth, or crisp but clear. The stock opamp is soft and slightly rough, I'd recommend changing it asap. The good news is most opamps are just a few dollars. Mids are very good with just about any opamp you can put in this amp. It's warm nature makes sure mids are well represented. Quantity is always just about right, though clarity and texture vary by opamp. Bass is also opamp dependent. The stock is somewhat wooly and boomy with too much mid-bass. The popular OPA627 class A mod is said to be very warm, but smooth and relaxing, so I would expect better quality bass, but still a good amount of it. The LT1364 is tighter in the bass and slightly less quantity than stock, quality is way up.
Soundstage and imaging is mostly the same with all opamps, but treble clarity will have some effect. The LT1364 is very good for this as it is brighter and clearer giving a larger but still focused soundstage and sharp imaging. The stock opamp has a good size stage but can tend to be fuzzy on imaging.
Another popular opamp is the LM4562 which I haven't heard, but is supposed to be somewhat neutral, so with the M-Stage's darker nature, with the LM4562, I'd guess it is just on the darker side of neutral while the LT1364 is it's opposite on the brighter side.
