The PL30 is slightly mid-forward in nature. If you like that signature it's not at all surpising that the PL50 sounds lacking. The PL50s are also thick-sounding and lack air, even compared to the RE0. Technically I find them vastly superior to the PL30 - more extended, flatter, faster, etc. But I ended up keeping the PL30 and selling the PL50 anyway. Can't argue with personal preferences.
Regarding upgrades, I find the RE252 to be a far more natural choice coming from the PL30 than the RE0. It's not as mid-forward but it's got the same airy feel and even better balance without losing out on detail or clarity to the RE0. Of course this comes at a price and a comfort sacrifice but that's just the way it is. You may also like the ViSang R02 though I have yet to finish reviewing them. Very airy earphone, almost open-sounding, and quite cheap.