It's funny that there's so much discerning opinions about the HP50.  I've read a couple negative reviews here and there of them, complaining it was too dark.  I read Purrin's post about liking the Beats Studio over it.  At the same time I'm also looking at the IF measurements and Tyll's impressions, and he doesn't seem to find that much difference between it and the FSP-- just like you.

Something has to be up to have such dramatic difference of opinion.

I've gotten quite a few messages on the Head-Fi boards about the same thing. I've had two pairs of them come across my desk: one model that was mechanically and audibly defective - that had a nasty midrange recess, shelved treble and a headband defect - and a replacement that sounds just like the one Tyll reviewed. I'm not sure if there have been problems with QC, if there has been manufacturing/batch variation, or what was up with them.

Something is rotten in the state of Denmark.

Very interested in these and would love to read some impressions versus the classic. I'm mainly and iem guy- would the isolation be good enough for airline travel? And would leakage be good enough for bedroom use to not bother the significant other?

Well I've got my reasons for not using iems when with my wife.

I think they (the professional at least) would do well for portable use.  The overall size is small and fits around the neck nicely.  They seal really well, but I'd worry about losing absolute neutral tonality in noisy environments, because bass will be lost a bit.  When I had the professional for a brief period, the headband's durability did seem in question, but it's nothing worse than what other portable headphones offer.  The finish of the professional lends itself to being scratch-resistant, so there's that.

I like the Professional considerably more than the RE-400 in all areas spare comfort.  The RE-400 is such a tiny footprint for an in-ear that it virtually disappears after a while and you forget that you have it on. Soundwise the Professional doesn't sound unnaturally cold and shouty like the RE-400 can a lot of times, it has a much less congested and more defined sound, and the treble is there unlike the RE-400.

I didn't really compare the two in-depth, but full-size headphones just seem to have a type of bass that in-ears can't really achieve.  I'd say both have similar amounts of bass quantity, while the Pro might be a little more strong-- don't quote me on that though.

Really the only thing that stuck out to me was just the timbre and natural quality of the Pro's midrange compared to the RE-400's.  I just don't like the midrange of the RE-400 all that much-- there's too much emphasis around 2khz and not much after that to me.  The Spirit Pro is more smoothed out in regards to its transition from midrange to treble and lower midrange to bass.