I haven't heard that many headphones, maybe a dozen or so, over the years and most of those have been lower cost, under £50, on ear and in ear types for portable listening and certainly nowhere near as many as Solderdude has heard. However, I have had a pair of HD650s for 15 or 20 years and I would never describe them as forward.
I really like the HD650 and I've been very happy with them, I always thought that they are just about right and I've favoured the lower cost portable heaphones that I thought sounded closest to them.
Some that spring to mind:
Koss Porta Pro - Similar overall balance, but a bit rougher around the edges. Hardly surprising, given the price difference.
Jays V-Jays - Simalar to the Koss, but more refined.
Philips Fidelio M1MKII - Nice sounding headphones, but I wouldn't recommend them as the ear pads fall apart after a few months and Philips don't supply replacements. Fortunately, the cheap copies I found on Ebay and have outlasted the originals and are still going strong.
Final E2000 - The nicest in ear monitors I've tried so far - I haven't tried any of the higher priced, balanced armature types.
This is all just subjective, I haven't compared measurements, frequency plots, etc., but I'd say that, like the HD650, they are all fairly full, rich, warm sounding headphones. I really don't like harsh treble or strident mids and I find headphones that have too much "up top" to be unpleasant, especially if combined with a "thin" bottom end. I never use any filtering, mods or EQ, I run everything as it comes using the bit perfect setting on both my home and my portable DAC.
Only very recently have I bought another pair of full sized headphones; a pair of Focal Clear, and what I like about them is that they are like the HD650, but "more", deeper, tighter bass, more presence in the mid range and more clarity all at the same time. However, I wouldn't want to go beyond the clarity of the Focals, I think any more would be too much.
That was all a bit of a long ramble, but unless their ears are very different to mine, I don't think anyone could dislike the HD650s. It's probably why they've been around, unchanged, for so long and why I held off trying to find something better for 15 years or so.