Responses will depend upon which Rowland amp you have.
Early Rowland amps used the 600 Ohm balanced input impedence standard and won't work well with some tube preamps.
The 6/8ti/9ti, the 10/12, and the 300 series amps are transparent and resolving -- they demand quality preamplification. These amps are fully differential balanced and benefit from a balanced preamp (that's one reason the top ARC amps work well with them).
The 201/501's are more forgiving of partner components.
I used two single-ended preamps with Model 6's, a Hovland with the amp switched to the higher impedence setting (18 Khz.), and an ARC LS-3, with the impedence set to the standard 600 Ohms. Both set ups were great.