The PMD-661 does not use more than 1A, and certainly not 1.5 A. just because the AC adapter can supply 1.5A, that doesn't mean that the PMD-661 is pulling that much. The manual says the PMD-661 uses 4.2 W (again, pg 64). Watts = Volts x Amps. at 5V, the PMD-661 is pulling 0.84 A (or 840 mA). and my guess is that that is the most it would be using (internal pre-amps, phantom power on, etc, etc). With the S/PDIF input, the deck is probably using a lot less than that.
I don't know how much power the USBPre2. but a tekkeon mp3450 powering the PMD-661 alone, I got 15+ hours of run time when using the S/PDIF input. that's why I sold the tekkeon, because I didn't need that much run time. now I just use the four internal AA's.