I owned these speakers for some time before moving up in the Dyn product line, and depending on your room size and preferred listening volume, your NAD should be fine. Jaxwired is correct in that these speakers do not have the brighter treble some Dyn models are known for, so if you decide to change I'd look for an integrated from NAD, Plinius, Simaudio or other good SS manufacturers with more power/current.
If you want something different, by all means try a tube unit. Tubes can definitely provide a different sound, just as the same album may sound very different when played on a turntable vs via CD or DAC.
If you want something different, by all means try a tube unit. Tubes can definitely provide a different sound, just as the same album may sound very different when played on a turntable vs via CD or DAC.