Cal,
You did it the right way by actually listening to various amplifiers in your own audio system. At that point you just have to trust your ears and 'know' what type of sound presentation you prefer. This stuff is so personal and that is what makes it fun and so interesting.
Stephen you have two excellent amplifiers for your Nenuphars and listening impressions can and do change over time (particularly if burn-in is a factor). The First Watt S.I.T.3 is a formidable component. I just believe that the Nenuphar has remarkable purity and transparency (without being analytical) and reveals what's fed into it within the system.
Charles