A Corda HA-2 MKII vs PPX would be the fairer comparison when taking the price into consideration. Further, meet impressions are to be taken with a grain of salt but for the good impression alone, I'd take the PPX with Solen 1 upgrades, Sylvania tubes over the HA-2 MKII (note: I spent about 1-2 hours with the PPX3). This preference, however, is just that and I don't consider the PPX superior to the Corda in general terms - both are excellent performers. Where the PPX lacked in tonal invisibility or even, in my opinion, detailing, it compensated via a richer tone, midrange bloom and a bigger soundstage. The Singlepower was also the slightly more laid-back and relaxing; more effortless at times, too. Surprisingly, the PPX did not falter in the treble as I would have expected from a mid-end tube amp. The PPX also had a pleasant bass: perhaps slightly more boomy compared to the Corda but more fun for a bass-addicted listener like me. The Corda on the other hand, offered a cleaner and more transparent sound although the PPX is not what I'd call extremely coloured - just a certain richness in the middle. Nonetheless, I think hat richness has too much of a positive connotation, almost euphemistic in the audiophile world, because not everybody prefers this "richness" and I certainly don't prefer the richness of an Earmax Pro over the tonal clarity of a Corda HA-2 MKII, at least most the time. I suppose, this richness has to be applied properly and in healthy doses according to one's tastes but I found the richness to be just right.
Both amps are great amps. The PPX offers the tuberolling capability, a tubey sound but without the prejudiced tubey-muddy sound whereas the Corda has the crossfeed feature, which I fancy quite a lot, and a solid-state sound without the often-associated solid-state graininess. For comparison's sake: In my opinion, both the Talisman T-3H (solid state) and Earmax Pro (tube) are inferior to the the PPX or Corda HA-2 MKII even though they are priced in a similar price range. Further, while being a stellar performer for its size, the SR-71 couldn't really compete against the HA-2 MKII either. Please notice the big price difference though: the SR-71 is not supposed to compete against a $800 amp. I'm only mentioning these references because often do I read over-the-top accolades without stating specifically "better than what".
Finally, I have no personal long-term experience with tube amps so I don't know how much hassle it is to maintain a tube amp. On the other hand, as a long time HA-2 MKI owner, it's always been plug-and-play without worrying about warm up, tube maintenance, dampers, heat, etc., that is issues I associate with owning a tube amp. Yet frankly, if I had to buy a $800 amp today, it'd be the PPX3 because there was something special - read: also extra, thus not necessariliy positive objectively speaking - to the sound. Certainly, this richness can be seen as a more natural sound but I think solid state amps are truer to the source. The Corda, nevertheless, had some problems, which the higher end solid states (Prehead e.g.) do not have, such as a certain strain in the sound reproduction. The PPX3 was not more resolving but it approached this technical barrier from a different direction and more or less eludes this problem by adding the tubiness. It's a matter of preference which trade-off you prefer because it's a trade-off not matter how you see it.
I don't think you'd be disappointed with a PPX3 but better audition it first because the HA-2 MKII and other amps are right up its alley. And with the danger of repeating myself, although I spent a lot of time with the PPX3, both alone and comparing it to the Corda HA-2 MKII, these are just meet impressions.