Been a while since I posted any impressions, the SigPro's were one of the best purchases I made and have since been looking for other equipment to build on their solid foundations.
Having recently bought a Grace M903 I can definitely say that the Grace pairs well with the Sig Pro, somewhat analytical and maybe even cold, the 903 has refined my Sig pro in to a more accurate and spacious reference headphone.
Straight from my iphone, ipod, the Sig Pro's are fun and warm sounding. Adding a Headstage arrow (4g) tightens the bass and refines the mids, with a similar sonic character only more detailed and luscious. Using the NJC audio monitor 1 (connected via USB to my MacBookPro) tightens the bass while maintaining the warm sound signature; I've come to expect from my Sig Pro's
Plug them in to the Grace and the Sig Pro's become an entirely different animal; it's as if the have matured (and yes they have been burnt in). The change in the sonic character which is only apparent when paired with a more analytical sounding AMP/DAC has calmed them, they don't shout to be heard anymore, they know what they can do and they don't have to slap you across the face with the lower frequencies to retain their excitement.
Paired with the Grace they are easier to listen to for long periods, leaning more towards the neutral side of the spectrum they are more revealing and spacious;
If you're looking for a good sounding Headphone AMP/DAC/PRE with lots of connectivity and you would like a more neutral sound from you're Sig Pro's I highly recommend the Grace M903