There's a lot to consider. 15 x 20 is a good size room, though can be filled with low power, high efficiency designs through high power low efficiency designs. Room height, layout (corners, openings, etc.), furnishings and such are a factor. The types of music you enjoy should also be part of the decision process.
We all have different tastes in music and certain speakers might perform better to your ears for different types of music, such as heavy rock, metal, light jazz, orchestral, detailed male/female vocals, instrumentals, and so on. My approach would be to suggest auditioning different sets of speakers to downselect which type best reproduces your preferred music. Once you have that decided, based on speaker efficiency and design, you can better experiment with what types of amplification would best drive the speakers for optimum music delivery. Then add the sources and room treatments. Others will likely have different approaches.
As you are unsure whether you might ultimately want tube or SS equipment, this thread might be better suited to be in General Audio Discussion forum, so that you get a balanced set of responses. Let me know if you'd like this thread moved there or are content with it remaining here in Tubes forum.