Those are my sources as well and I own both phones. Both drive the phones just fine.
The GR07 sound sig is a more balanced and monitor like and maybe just a hair more clarity than the FXT90. To me the GR07 treble is pretty smooth being neither dark nor overly bright with decent sparkle but no air. Mids are clear and a little forward. Distortion guitars have really, really great bite. The bass has little mid bass and mostly sub bass presence. They really have good sub bass rumble and texture. On the down side, the dynamic range is soft, which means less of a difference presented between powerful and delicate passages. I believe this also attributes to it's shallow soundstage depth. However it does have a very wide soundstage.
The FXT90 sound sig is more musical and layered. The FXT90 treble is brighter with more sparkle than the GR07. Mids are also a little forward and clear and can be more intimate when necessary. The bass is equal parts mid bass and sub bass, so while it doesn't have quite the same rumble as the GR07, it does have more punch. The FXT90 is very dynamic and very powerful in it's presentation. It transitions easily from delicate to powerful passages. The soundstage is only average in width but has excellent depth due to the greater dynamics, layering and reverb the dual drivers produce. Instrument realism is superb and rivals that of it's big brother the FX700.
I think the FXT90 is a little more comfortable to wear and I like the cable better, as it is supple and easy to deal with. The GR07 cable is stiffer and tougher to deal with but its housing is comfortable in it's own right, considering it contains an 11mm driver.
You really can't go wrong with either. If you want a more monitor like presentation with more sub bass rumble, go with the GR07. If you want a more powerful and musical presentation, go with the FXT90. Or be like me and buy them both.