Ok I got myself some HPAEC750 from Juno also (19€, free shipping in Europe).
I have two 12€ "FA-004 clones" (Ace MP-68, MC Crypt DJ-50), which sound the same and have the same pads. Also two headphones outputs on my amp so it's simple to switch and compare.
I would not say the bass change is slight at all. There's especially more subbass, overall something like +3dB (compared with EQ). It makes the sound a bit muffled, requiring more volume. I cut thumbprint sized holes to the Shure fabric to try to compensate a bit.
Hard to judge soundstage size, perhaps just slightly larger. Comfort is quite nice now.
Overall warmer and smoother sound, makes quite fun "30€ headphones", I like it. Of course it isn't for anyone wanting good vocals etc, stock already is a bit too bloated for that..
Just to clarify, you're saying the Shure replacement pads are not a precise fit? Do they slide around a bit? I'm still looking for the ideal replacement pads for these. I can't see getting the oval shaped Beyer pads either. I've seen photos and the just look wrong on the FA-004.