Having lived for so long with my K701s as my top can, I find the Prolines very engaging. The PL650 I originally bought was unlistenable for me in stock form. After many modifications to address the partiality of their manufacture, they are a really great monitoring can now with a surprisingly large soundstage for a closed can. Vocals and stringed instruments are superb. I think they are more impactful and punchy than the K701s. The K701s are still more refined, less grainy, but that element of the 650s has improved quite a bit with burn in and the mods I did.
The PL2500s, being open cans are more appropriate to compare to the K701s. I don't have enough hours on my pair yet to pass final judgment, but after the mods I did to the 2500s, they are quite wonderful sounding. Pretty much everything that the PL650s do well, the PL2500s do better. The bass on the PL2500s is punchier, more impactful, faster, and deeper than that of the Senn HD600/Cardas.
Regardless of ad copy, the drivers themselves in the 650s have very similar sound signatures to those in the 2500.
The biggest difference being in the loading of the driver, as the 650s are closed and the 2500s are open. There are bass reflex type port holes in the baffle for the 650s to relieve back pressure where the backs of the 2500 cans are open to relieve back pressure. So the 2500s are analogous to infinite baffle speakers. The ear pads are designed to completely seal on the 650s and designed to be porous on the 2500s.
Using the 650 pads on the 2500s results in a waay overly bassy sound, while using the 2500 pads on the 650 results in a very thin and anemic sound. This is as expected, given the design differences of the two cans.
I have listened to nothing other than the Prolines for the past few weeks and eventually I will throw on the 701s again to see how things have changed WRT direct comparison.
There is something special about Prolines!