Upgrade from Silverline 11

I was going to suggest the same thing - stick with Silverline since you like the 11's - he makes excellent speakers all around. That said, I don't know that the step up to the 17's would be that significant for your having owned three different pairs of them. They are certainly excellent speakers and for monitors give you a pretty great low-end, but if you really want to improve on that aspect of performance, why not go to a floorstander? The Silverline Sonatina would be a great choice, or the Sonata if you can find one in your price range. You will loose a bit in the imaging department going from monitors to floorstanders, but you will gain quite a bit in terms of low-end over going to the 17's in my experience. One footnote - do be careful in purchasing used speakers that the seller has a good reputation and either original or professional packing, and with shipping insurance mandatory. Better to buy local whenever possible. The downside of the older Silverline models is that some of them no longer have drivers available should one become damaged :-( Still, I would consider a floorstander for what you are asking for in improvements in the low end (which do necessarily come with a high price tag to get that really great - but used you might be able to find something very good). Certainly an SR17(.5) would be a step up as well, I just don't know how big a step up in terms of perceived differences.

I've been offered Silverline 17's from Audiogon member that lacks references (rating). It could be alot of money to lose. Thanks.

SR17's have gone through at least four significant changes over the years. There were three versions of the original SR17's where Alan changed specific aspects of them (drivers, crossover, cabinet maker and internal bracing, in the very last iteration as I recall). The biggest differences were between the first and the second/third versions (not as much between the latter two). Then there's the 17.5 which succeeded all of them and is supposed to be better still. I have not heard one so cannot comment there.

Personally I would be very careful with buying speakers from someone with no track record. Yes, it's a lot of $ to loose and not worth the risk IMO.

Good points about overloading a small room - I'd missed that point in the OP. I've used Sonatina I's and Soliloquy floor standers in pretty small rooms with some treatments and had good luck there, though I'd say the Soliloquy did better in a larger space. The Sonatina did fine in both. Neither are the last word in low-end reproduction, but both were pretty good and sounded quite a bit more substantial than SR17's do. It is certainly something to be careful of though. It will depend on the room, contents, treatments, volume you like to listen at, and of course the specific speakers themselves. The good part is that in a smaller room you can get some bass reinforcement using smaller speakers. AudioNote was a good suggestion in that regard, since they were designed with corner-placement in mind, though I don't know how they'd go with your SS NAD amp. No experience there. It probably goes without saying, but if you do use monitors their performance is significantly enhanced by the use of a good stand (as opposed to putting them on a shelf).