Rather than pay to have your unit upgraded to a Signature or buy a second P.S., I would consider selling your unit and buying a Signature with or w/o the 2nd P.S. on the used market. I saw an Io Signature with 2 P.S.'s (no volume controls) listed here several months ago for $6k. Now that's a sweet deal!
In the meantime, do a search for some of Albert's threads on tube rolling for the Io and Callisto. To invest in $500-1000 in his recommended tubes will transform your unit to a level that far exceeds its capability with stock Sovtek tubes. When I did this it took the Io to a level far greater than the Io had done over my previous BAT VK-P10. So my gut feeling is that this could be the most dramatic improvement of all here.
John
In the meantime, do a search for some of Albert's threads on tube rolling for the Io and Callisto. To invest in $500-1000 in his recommended tubes will transform your unit to a level that far exceeds its capability with stock Sovtek tubes. When I did this it took the Io to a level far greater than the Io had done over my previous BAT VK-P10. So my gut feeling is that this could be the most dramatic improvement of all here.
John