I use my UE Triple Fi 10 with N900 and has a very good sound quality. But the volume is kinda low though.

Not sure about GS2. If you want the best audio quality, go for GS1 with voodoo sound kernel. The sound is amazing quality and powerfully loud. Thats the best audio reproduction for a phone out there.

However voodoo kernel doesnt work with SG2 as it has yamaha chipset