String guages for slide dulcimer
General mountain dulcimer or music discussions
The onlty slide player I've ever seen was at Cullowee years ago when I was in the Traditional track. One of the teachers -- Norris Bennet -- was a member of the Ebony Hillbillies, the famous black NYC string band. Slide dulcimer certainly is an interesting sound. As Wally sez, you can try other gauges but there are scientific reasons why you can only go so thick or thin on string gauges for a particular VSL.
Something you might want to try is using wound versions of the mid and melody strings, to 'fatten them up' closer to the diameter of the bass. Juststrings.com sells Phosphor Bronze wound 12 and 14 gauge Acoustic Guitar strings for $4-$8 depending on brand (and string brand is irrelevant).
Conversely, you can try using a plain steel bass string (we traditional dulcimer players do this all the time to help us get that 'high silvery' tradtional sound.
Also, the bass string notches in the nut & bridge could be filed just a very tiny bit deeper to bring the top of the bass string down just the tiniest bit.