Audio Research REF 110 or Ref 75

You'll get more power, of course. More importantly, though, it should be cleaner sounding. The VT100 is great, but it does have a bit of an electronic haze in the higher frequencies. Honestly, its not very bad and I didn't even notice it until I put it next to amps like a VAC 30/30 and an Ayre V-5. Other than that, just make sure your preamp is up to the task. If you already have a great amp like your VT100, quite often a new preamp can be a better upgrade.

Its a tough choice because both pieces are very good, but there is still room to upgrade. If it were my choice, I would go with a new amp if I thought I didn't have enough power, or some other issue with the amp that you know you don't like. If you have no issues with the amp, a new preamp will usually make a bigger difference. Either way, you should seek more input. I have a lot of experience with your amp, but not so much with your preamp. Given that, I can't make a fair comparison. Overall, you have a very well matched system. Wilson/ARC go very well together if done right. Are you working with a dealer? That would be a big help.

"07-29-14: Mitch4t
.
Zd542, Lak & Vdosc, Would a subwoofer be a possible solution
in this scenario?"

In this case, I would say no. The Watt/Puppies can go pretty low all on their own. The real issue is having a 15 watt amp. It should be OK with the mids and highs, but there's no way an amp like that can drive the woofer properly. That also makes it very difficult for the sub. If the sub can pick up where the Puppies leave off when driven properly, that's one thing. That's not the case here. To get the sub working well, it will have to take care of frequencies that Puppy should be dealing with. It shouldn't have to do that. You would have to try it to be sure, but my guess would be that you would hear the sub strain/distort because its working outside its range, or you can let the sub work properly which would almost certainty leave a gap in the lower frequencies that neither speaker is producing. How bad would it be? I can't say for sure because there is a lot of factors involved that you would have to just try it to find out. Also, you would have to figure in the subjective part of the equation. For me personally, the highs have to be just right and I can live with less than perfect bass. It can be the exact opposite for someone else.