Now for a shocker, I'm not happy with my HE-300s anymore... I was completely satisfied with them before the stack. I am being completely honest saying that these just dead end and don't scale. I feel like the amp is wanting something better to work with and the HE-300 cannot keep up. I will be looking to get HE-400s very soon now and enjoy that sweet soundstage/speed/bass/detail upgrade.
Well, if your HE-300s were doing something right before, you might still want to hang onto them.
One thing I've learned over the years is that system synergy is king. You may have a system comprised of very highly rated components. But if they don't work as a complimentary team, the results can be severely lacking.
On the other hand, you can put together a system of moderately priced or even inexpensive components, ones that work very well together, and the results can be a lot, lot better than you would expect them to be.
For example, I used to work in a high-end audio boutique in Manhattan (what a ridiculous term for a stereo shop, I know). We had expensive, highly rated lines, as well as moderately priced ones, and some "budget" ones.
The one system pairing that the staff was contstantly amazed by was a Cairn or Creek DC player hooked up to a Cairn or Creek integrated amp, driving Epos 11 speakers.
The music from this system was so communicative and emotional. One local guy (who, incidentally, would come in for long listening sessions but never bought anything) actually cried when he listen to his music on this system. That was a first for me, but it confirmed what we were observing about system matching.
Now this isn't a budget system (I think the whole getup was around $2400 for the CD player, amp, and speakers at the time), but it was as musically satisfying as other systems we had that cost 4, 5 or 6 times as much.
Incidentally, all of these components performed well with other amps, speakers, etc, but this particular combination really elevated the performance to a special level. System synergy.
Regarding your HE-300, if there's something you really like about them, you may love them a lot more with a complimentary amp, dac, etc. that pairs well with them. It doesn't sound like the Vali is part of the equation with these phones.
On the other hand, you could build your system off of the Vali, and see what headphones, dac, etc. pair well with it.
Please read the thread for people's impression of the HE-400 with the Vali. I didn't find it a satisfactory pairing, myself, but others have had success with it. Still, if you get a HE-400 and find that doesn't quite do it for you on the Vali, it's still a great headphone to have.
Good luck!