Don't have the HP50 to compare against, but i can offer a quick companion between the K545 and P7.
The first thing that struck me in the K545 was the bass, as it was more impactful that I had expected, which was a nice surprise as I have heard that AKG headphones tend to be bass light. That said, the bass was never overpowering , although it could be a tad tighter (the P7 bass is tight and well controlled for me). Doesn't extend as deep as the P7 as well, but still good overall.
After getting over the initial bass surprise, the mids started to come out nicely. The mids are quite rich sounding, with a similar tonality to my AD900x, which is an open back headphones. I'd imagine the mids being not as forward as the Momentums (based on impressions of listening to Momentums in store), but definitely not as backward as the P7s. This also gives the impression of a large wider soundstage than the Momentums. Soundstaging sounds pretty much on par with the P7.
Treble is quite well extended, slightly rolled off compared to the P7, but nothing really to complain about.
Fit and comfort wise, I was happy to find that it was extremely easy to get a fit with this headphone. Clamping force is lighter than P7 and exerts no excessive pressure on your ears. However, the earpads are slightly thin, so some people may find parts of their ears touching the inside surface of the driver section. Not a major concern though. The K545 feels a lot lighter than the P7 too. Isolation is not as good as P7 due to thinner earpads and lighter clamp force. While I have no major comfort issues with the P7, I would imagine this being a more comfortable headphones for most people.
Overall i think K545 offers good value, considering it's almost half the price of P7 and about 100(?) cheaper than HP50. Not too sure about prices you can get, but this is based off my memory from Amazon prices. Build quality is pretty good too, with a good mix of plastic and metal.
Nice impressions, i felt they were more "fun" > hp50s. Is it as transparent and clear? No. But they are no slouches in that department.
I found the comfor was better the earpads are bigger. Abeit circular.
The build felt was better, more robust feeling. And their definitely more slimming on the head. The hp 50s square headband is uber comfy but walking around with that is just odd to me.
Isolatation i thought hp50s won out there