Honestly I would just continue to use the Porta Pro. Of course you need a corrective EQ to fix the high frequency roll-off, but the beauty of them (other than being open backed) is that they’re on-ear as opposed to in-ear or over-ear.
I posted a dedicated thread about the KOSS Porta Pro the other day but deleted it shortly after. I thought peeps might think I’m taking the piss with them having such a crazy roll-off, but with a corrective EQ for mixing I really like them.
Also I was a teen in the 80s so maybe I’m biased, but I absolutely love the physical design as well. You can buy the same model today as was launched back in 1984 (the one with blue driver vents and a standard stereo cable).
Bert, Grover and Santa Schwarzenegger made a nice review video about it and even threw in some history. It’s a fun half-hour and you just might find you no longer wish to replace them.
Seriously, just stick a corrective EQ in there, that’s what I do and I freaking love the things!