I currently have the QX4600 - in a gun with Mk1 stroke it's up near 11.7 ft lbs with no top hat and no spacers....just custom guide. So there really is no spacing option. It would be nice to be able to adjust pre-load some more...I can't go up with this one. The piston is lightened as well.

Here are some other Maccari springs that I can easily source here in the U.S. (some may not fit the TX factory guides though and will need custom ones):

QX4600 spring 32 coils x.120" wire - ID .560 set
http://www.airrifleheadquarters.com/...88/7905117.htm

R6 Ultra XLD spring 30 coils x .120" with .530 ID
http://www.airrifleheadquarters.com/...88/1979288.htm

E9870 Euro Universal spring 39 coils x .120" wire 480" ID - 13" OAL
http://www.airrifleheadquarters.com/...88/6825218.htm

E8700 mini spring .105 wire, 29 coils, ID .555" 10" OAL
http://www.airrifleheadquarters.com/...88/5730289.htm

Merlin XL spring - Square wire, figure it at .116" average, .509 inch ID., 37 coils and 12.2" plus unset.
http://www.airrifleheadquarters.com/...88/4452198.htm

E3200 spring 32 coils, .110 wire, ID .540 inch - Unset 12" OAL
http://www.airrifleheadquarters.com/...88/7908088.htm

Merlin Mini Mag spring 29 coils x .110 round wire x .545 ID
http://www.airrifleheadquarters.com/...88/9682441.htm