To better understand and assist: What amp/pre are you currently using? What do you like/dislike about it? Have you demoed a tube amp in your system? Why do you think tubes may be a good choice?
If your Audience 50s are 4ohm, 88dB you may be happiest with a late-model, higher quality used (say, Audiomat Arpege) tube integrated for $1,000 - $1,500 that can drive and control your speakers. If you don't require higher listening levels on dynamic music, there are several
quality(?) Chinese tube integrateds out there for $1,000 to $1,500 (new). Just know that some are unsupported and the longevity may be questionable.
If your Audience 50s are 4ohm, 88dB you may be happiest with a late-model, higher quality used (say, Audiomat Arpege) tube integrated for $1,000 - $1,500 that can drive and control your speakers. If you don't require higher listening levels on dynamic music, there are several
quality(?) Chinese tube integrateds out there for $1,000 to $1,500 (new). Just know that some are unsupported and the longevity may be questionable.