You'll only have a problem with volume if the power amps are very insensitive. If not all you are doing with an active pre is adding more gain you don't need then attenuating it again. Depends on what active pre-amp you were using before, but unless it was a very good one there should be a notable step up in quality of sound, particularly with digital.
Problems I have encountered with some passives are a lack of impact on what should be hard transients, like snares, making them sound too soft; and a perceived lack of bass. The passive may be more sensitive to the interconnects used than an active, too. Keep us posted on how you get on.
Current Lash Up:
Technics SL1200 with Sumiko h/s & Nagaoka MP50 * Firebottle valve MM phono stage * Parasound CDPi1000 * NVA P90SA passive pre / Krell KSA50S Power amp * JM Lab Electra 926 loudspeakers *