Amp or speaker first, upgrade

I own the Vandy 3a sigs and enjoy them immensely. I have no experience with the Definitive speakers, so I cannot give any comment to their sound
For me, I would consider upgrading the speakers first.
If you think you will go with Vandy's, I suggest moving to the McCormack amps and preamps as they mate with Vandersteen speakers nicely. They also can be upgraded by Steve McCormack, should you want, in the future.

in your particular situation I would do the speakers first

Speakers "usually" have the most impact.

Absolutely speakers.  I have owned Def Tech speakers, and when I upgraded them to a far more sophisticated speaker (Rockport Mira Monitors) I was extremely pleased.  Overall I was not particularly happy with my def tech speakers (I had the Mythos STS Super Towers) and I kept them for only about a year and a half.

The Krell amp you have has more clout amongst integrated amps than your def techs have amongst speakers.  I'll bet that Krell will really impress you with some good speakers.  In fact, I would recommend putting all of the budget for the amp and the speakers towards the speakers, and keep the Krell for a good while longer.  Sure, you may upgrade it eventually, but in my humble opinion, you'll get more improvement in the system with an outstanding speaker than you will with a speaker and amp upgrade sharing funds.

these may or may not work for your budget or your interest, but there is a pair of Wilson Watt Puppy 7s in excellent condition on agon that have dropped in price of more than a grand over the past few months.  At 4 ohms, your Krell's 300 watts should be fine with them.  I auditioned them (not the exact pair) and was very impressed!. I wanted to buy them, but I was drawn to a different pair of Wilsons closer to home.  As I said, these may or may not be for you, but worth a look and a ponder anyway.

As someone who owned Wilson's,  I  encourage you to listen to Aerial 7Ts and Revel F208's depending on your budget.