Finally, some contenders.
Craigster, please do the world a favor and do an M-100, PRO 500 and HP-800 shootout, with comparisons. Or at least with the ones you have. If someone else can provide M-100 and PRO 500 comparisons, we can deduce the rest.
Finally, some contenders.
Craigster, please do the world a favor and do an M-100, PRO 500 and HP-800 shootout, with comparisons. Or at least with the ones you have. If someone else can provide M-100 and PRO 500 comparisons, we can deduce the rest.

yeah those hp-800 are a little huge

It's interesting to note that the PRO 500 sounds at least as good as the M-100. It's a shame that they aren't very popular on Head-Fi.
Still, for the moment, I'm going to pass on the PRO 500 - I'm not really a huge fan of its design.
Thank you for answering that burning question for me. I was about to pull the trigger on the PRO500 and would have returned them if they weren't a big sonic upgrade from the M-100. Although, I'm not sure how that would be possible 
I know it should really be about the SQ than the looks, but likewise, if I had to walk around with the PRO 500 on my head I'd feel like a right ****.
Good thing I only listen at home. The sound is what's interesting, would be funny with modded Hello Kitty headphones with high quality drivers in them just for the laugh. :P
I haven't heard the PRO 500, but from reviews, they are comparable to the M-100 sonically with a bit more upper bass/lower mid. They are also $100 more, heavier and look too much like Beats, so I will likely pass although I want to hear them.
Regarding the M-100 and HP-800, I have been A/Bing and find they actually have a similar sound which surprised me. If anyone can compare their frequency charts, I think they would have some similarity, particularly in the mids and highs. The M-100 is still the best sounding closed portable under $500 I have heard. In fact, I have a new appreciation for the level of M-100's bass when comparing to the HP-800 because they are very bassy and the M-100 holds its own. Bass is deep, accurate and powerful with both; bass is tighter on the M-100 and the bass presentation on the M-100 is less intrusive into frequencies where it doesn't belong which makes them superior for all genres. The intrusiveness of the HP-800 makes them better basshead cans that are genre-specific like EDM and hip hop.
The HP-800 is a non-portable, slightly bassier, less refined M-100 which should be considered high praise coming from me. The HP-800 is solidly constructed with aluminum cups and offers incredible comfort but they are huge and don't fold. If your budget only allows $150 and you are a basshead who doesn't care about portability or looks, you will be very happy with the HP-800. When I say looks, they are a nice looking headphone with a simple design that works well for home use, but if you are single you wouldn't be doing yourself any favors wearing them in public. In contrast, the M-100 could be a conversation starter. If your budget allows for the $300 M-100, you will gain refinement, midrange detail, great looks (subjectively) and the best portability available in a circumaural headphone also with excellent, deep and powerful bass.

IMO, there is now a clear-cut choice for bassheads looking for a closed circumaural in the $150 range as long as portability is not an issue. They are incredibly comfortable, ear-shaking bass AND great sound across the range. I'm having a blast with these right now. Imagine an M-Audio Q40 (same level of bass) with more midrange detail than Q40 and smooth extended highs. I'm not saying these are better than the M-100 which is still my #1 audiophile headphone. I'm saying I'm hearing the best basshead headphone sound I have ever experienced with the HP-800 and my FiiO E11 with bass boost on #2 and volume at #6.
I still have my doubts but yea I will be keeping an eye on these. That it would have better midrange detail than Q40 is hard for me to believe as I love the mids on Q40 (and I've tested several AKGs and Sennheiser HD598 for example and the Q40 still compares very well with my setup) but I think it's due to my carefully adjusted extra paddening in the cups and swapped cable and the absolutely wonderful synergy you get when pairing ZO2.1 with the Q40 as well as the many hrs more burn-in that makes me see Q40 from a bit different perspective than you. :)
My plausible Q40 replacements looks to be right now either NAD Viso HP50, M100 (but I may wait for XL pads or lower pricing as I feel the european pricing is slightly too high) and Nuforce HP-800 (but my fear is that midrange isn't enough in-your-face which I get with Q40 + ZO2.1).
For me it's extra difficult to find a replacement due to loving the sound of ZO2.1 but it's got hissing issues with sensitive headphones (and obviously IEMs) why the ZO2.3 quickly surfaced but with 64 ohm or a little above it works perfect and I can't quite get the same sound I love with ZO2.3 with most headphones, it depends obviously how they sound out of box too but yea it's a tricky situation, perfect would be with a 64 - 80 ohm headphone with the sound signature I like, ie. noticably ~8dB or so boost in bass (centered around 80Hz for equal presence in midbass and subbass) with an otherwise "in-your-face" sound with neutral highs and very slightly more forward mids in comparision to highs so there's a slight warm tilt. Furthermore I hate resonance peaks in the midrange and will quickly spot such and I hate grainy highs, rather a bit on the softer side highs than too sharp/metallic sounding as well. Feels like I'm so picky about the sound I want to experience. lol
EDIT: Can you measure the inner-diameter for the pads on Nuforce?

My plausible Q40 replacements looks to be right now either NAD Viso HP50, M100 (but I may wait for XL pads or lower pricing as I feel the european pricing is slightly too high) and Nuforce HP-800 (but my fear is that midrange isn't enough in-your-face which I get with Q40 + ZO2.1).
I believe any of the three will be a good choice.
EDIT: Can you measure the inner-diameter for the pads on Nuforce?
1.75"/4.5cm- very spacious for my ears. The cups are K550 size, but much better fit.

M-100 or PRO 500? Also, I can purchase the PRO 500 at 15% off today. Any additional comparisons for me?
Thanks, 4.5 cm, hmm smaller than I had hoped, the Q40's diameter is 5.5 cm and fits fine, I have pretty large ears.
Believe it or not, you have helped confirm my answer. Since they are comparable, I am going to stick with my M-100 and put my curiosity aside for now. Too many good headphones out there and not enough $$$ to own them all.