If you have to buy a tubed preamp to try your experiment, I'd suggest not doing so. It's not gonna get you where you want to go. Find some new amps. Digital amps are like Twiggy, or Paris Hilton. No curves. Tube amps are more like women with curves. Ever see a silicon chip? Lots of right angles. Ever see a tube? Curves, curves, curves... :)
BTW, not all digital amps sound the same, and you haven't mentioned which you own.
BTW, not all digital amps sound the same, and you haven't mentioned which you own.