Foreword
I bought these headphones in February this year from fellow head-fier @dieslow, and got them after about three weeks, courtesy of Swiss customs. After reading Steve Guttenberg's rave review on Cnet, I was all too excited to try them out on my new iDSD BL. Boy was I disappointed! They sounded dull, veiled, flat to the point that my Fidelio X2 (a VERY good heaphone, btw, especially considering the price) was miles ahead of them in terms of everything, really. So I decided to put them on the chopping block, pretty much for the same price I had paid for them, and recover the investment. No such luck. But really? More of that at the very end of this review.
Enter Spirit Labs. This small but very professional Italian outfit is run by founder Andrea Ricci, mainly intent on heavily modding Grados and Beyerdynamics. I bumped into his Facebook home page while I was trying to get rid of the TH500RP and saw in a corner of the page the name "Fostex" among the brands of cans Spirit Labs would mod, so I hit them up and explained what was wrong with my headphones, imo. Andrea listened carefully, then explained to me what he thought could be done. It didn't take much to convince me at that point, also because the price he quoted for the whole job, new premium cable included, was a very affordable EUR 140. So I shipped them out. What follows next is your typical ugly duckling story.
Pre-Spirit Labs mods
I got the headphones in really excellent conditions, just as described by @dieslow. Build quality and presentation are really outstanding.

Soundwise: overflowing and uncontrolled bass, mids not well defined and "foggy", rolled-off treble, meh soundstage, needed to be played really loud to get something out of them.
The Spirit Lab mods
Before the mods:



The famous RP drive


The mods


Basic design change, from open to closed back.
Removal of the mat covering the drive. Total internal recabling
Removal of the mid paper layer behind the drive, baffle/driver armoring

Final product with new cable.
The final sound results
Bass:
Powerful, tight and detailed. Clear and rhythmic mid-bass.
Mid-range:
Open and transparent, a major improvement over the pre-mod version! Can be played at low volumes and still retain clarity and detail.
Mid-highs and highs:
Crisp and well defined, without ever becoming sibilant.
Soundstage:
As odd as it may sound, there is a much better soundstage now, after being transformed into a closed design, than there was before as an open design. I believe this may be due to the vast improvement in transparency and clarity.
Power:
Before the mods, I really needed to drive them hard with my iDSD BL in full turbo mode, and the result was still questionable to say the least. Now, the BL drives them effortlessly (in turbo mode), but I have achieved amazing results in terms of their ability to play real loud or at low volume by using the headphone stage of an icon of the '80s, the Audiometric A10P/Adcom GFP1 semi-pro preamp, which to this day I consider one of the best preamps, bar none. However, I'm not even thinking of using them on the go, the BL being the maximum portability you can get with these cans.
Conclusions
So, thanks to these rather extensive but inexpensive mods (take a look at the cost of the Lawton mods to see what I'm talking about, and they are d.i.y!) the ugly duckling has become and elegant and powerful swan. Boy am I happy I didn't get to sell these cans when I wanted to! If I had, or if they had been sounding reasonably well, even though not amazing, I would never had them go through the Spirit Labs treatment and therefore I would not nearly enjoy them as I am enjoying them now. Yes, the cable is a little heavy, and my ears get warm after some time, but I still can't take them off after several hours!I am therefore renaming my headphones the Spirit Labs SL1000AR. Thank you Andrea!