I've never heard the D5000, but as you've intimated, the RSA house sound is warm, where the Quickstep is spectacularly neutral. Bass impact will vary with the efficiency of your headphones and the supply voltage to the Quickstep (up to 15VDC). With IEMs or really efficient headphones, an increase in supply voltage (above the 9V internal battery), would likely make no audible difference. I really don't know how "hard" the D5000 is to drive, but the LCD-2, for example, really comes to life with more power - which translates to improved bass.
The D5000's are easy to drive at 25 Ohms. I just want to make sure the Quickstep has good, powerful bass impact as I know the RSA amps are renowned for their powerful bass but don't want to add a warm amp to an already warm headphone.