Within your budget, there are a couple of amps I could suggest to drive the S1’s. The first is an Australian-made amplifier which I first heard at the Australian International Hifi show last year, the Holton Five-Zero-Zero power amp - https://holtonprecisionaudio.com/collections/fully-assembled-amplifiers/products/fully-assembled-and... The Holton is very well made, has a lot of power & punches well above it’s price point. That amp will drive the S1’s with authority & has a high input impedance which would be benight to any tube preamp.
The other amp I could recommend would be a 2nd hand Modwrigth KWA-150SE. The Modwright has good power, sounds tube-like, is very quiet and has excellent inner detail. It is a beautifully made amp. The Holton however has more power & would provide better control, which in the case of the S1’s is very important. You would also be buying new in that case vs 2nd hand. My 2c.
The other amp I could recommend would be a 2nd hand Modwrigth KWA-150SE. The Modwright has good power, sounds tube-like, is very quiet and has excellent inner detail. It is a beautifully made amp. The Holton however has more power & would provide better control, which in the case of the S1’s is very important. You would also be buying new in that case vs 2nd hand. My 2c.