You will need a pre-amp. A cheap alternative would be to get a receiver with re-amp outputs like the Onkyo TX-SR801.
It has Net tune, which is a network mp3 client like the Slimp3 but built in. You probably could find a deal on the net. These units retail around $600 to $800.
Just a thought, after you buy an amp say $400.00 and the slimp3 unit $200.00 and then a preamp $???.00. You’re looking at least $800.00 or more.
I personally would go with the receiver. The TX-SR801 does have pre-amp out puts so you could add an amp later.
Just something for you to think about… I too like the amp route.
Tom