Why not a bookshelf like the Golden Ear AON 3 (well below your budget) ? I have heard other speakers from Golden Ear , and was highly impressed. These stand mounted speakers use the same excellent folded ribbon tweeter they use is their floor standers, and claim bass extension down to 38Hz.
Also, as others have posted, you might look for a pair of good, musical subs, like smaller RELs, Golden Ear subs, JL Audio subs, or a Vandersteen 2Wq (provided your Silverlines extend down to 40Hz, and your amp has a tape loop). You can keep your Silverlines, which probably have mids and highs as good as anything you can find within your budget,