@thecarpathian I would agree with most of what @yakbob says, though I’ve never really had issues with KT-150s being noisy or dying. Below are my additions.
Overall, the EVO series is a very modern tube amp design and sound. This means that the amp does not have typical coloration that one might have with a tube amp. It does have some bloom, especially depending on tube types used, but one would never mistake it for being as colored as a classic SET amp. I actually found it quite difficult to get a lot of bloom with the Primaluna. This is neither a strength or weakness, though some may perceive it that way - it really just depends on what suits you best.
With good quality tubes, I find the EVO series to come quite close to Audio Research, but for much less, especially if your speakers are easy to drive. The amps are dynamic, with decent warmth and good harmonics, again, without being too colored. I do prefer the sonic and build quality of Audio Research which I will get to later.
The build quality is very good, and it’s versatility in terms of tube rolling options are very fun and make the amp worth playing around with for years. Also, with the ability to switch between ultra linear and triode modes, you can experiment the sound you like best for particular recordings. It is a fantastic value for its price given the entire package.
As for weaknesses, I do think there are a few. First, I don’t think the amp delivers optimal current to control and drive speakers like endgame amplifiers do. Combined with the 300 putting out a max of 50wpc, which results in less headroom, it can produce a sound that is softer in some areas and harder in others and ultimately sound less resolving and organic than desired. I do like the 400 better in this regard because of the power and headroom and like an octet of good EL34 or 6CA7 vs a quad of KT150. Also, I’ve never gotten Primaluna to deliver the clarity and decay in the upper treble to depict the air, space and depth as I have in other endgame amplifiers. This is where I find Audio Research to excel, both with the current delivery, control, clarity/holography, and organic delivery. But here we are comparing a $4-5K integrated amp to one double to triple its price, and when I mention other endgame amps, I am also referring to ones that fall in the $20-40K range, so I am being very critical in this sense.
I guess the last weakness is that for the PL to sound its best, you should plan to spend upwards of $1000-1500 on tubes. The stock tubes are really, really mediocre.
With all this in mind, I still highly recommend the PL for its price point. It is a very versatile amp of very good quality, especially for its price point. I’ve owned them many times and still have one in possession for demonstration purposes against other amps. I am not a dealer for them, and I don’t plan to be. Most of the amps I sell cost much more, but if I do find someone who only wants to invest $6K or less on a tube amp that is not low-powered, I don’t hesitate to have them consider PL.
Hopefully you find this helpful. The big question for you is, what speakers are you using, and why the 300 vs the 400?