''Hissing'' from speakers....cause ?

Thar's right Plato, the Monarchy is passive, the Sim active. Never considered that, thanks.
It's noticeable when I'm close to the speakers, or if my small listening room is very quiet.

It appears (correct me if I'm wrong) that you went from a passive preamp to an active model. Passives are, well, "passive" and should have no inherent "hiss," while all actives have some degree of hiss, some more than others, but if it's a modern unit then the hiss should only be heard with your ear close to the speaker. This is normal.

Probably only way to "get rid of the hiss" would be to go back to a passive preamp... If the hiss is loud, meaning you can hear it at your listening seat, or is louder in one channel than the other, then you may have a problem with the preamp that would require servicing.

Tubes hiss. Everything else hums. Good equipment (in proper working condition) is inaudible.

I had exactly the same situation that is audiable with some amps and with some amps it's less audiable. When I replaced tweeter, everything had straightened out. I even had come to the point when I had to block the tweeter with soft paralone so it doesn't hiss to loud and listen most of top end from another speaker but it was temporarily to save my sencitive ears.