In my opinion the Signature is a more important upgrade sonically than the second supply.
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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.
Albert has the most experience with the Io in its various iterations. I'd recommend following his advice if you chose to upgrade with the unit you have. The second power supply improves the sound in different ways than the Signature upgrade.
Here's how Jim White described for me the differences he heard between the standard Io and Io Signature, and with and without the second power supply option:
The Signature provides the following benefits: