Greetings, I'm new to tube amps. i have a small, cheap 5W (1 12AX7, 1 6V6GT) that's only about 18 months old. Initially I'd let it warm up for 10-15 minutes and it was great. Over time, it's been taking longer and longer to warm up. Even after 30 minutes, the volume is super low and the tone isn't right. Eventually (now taking well over an hour) the volume and normal tone kick in abruptly and from then on it's fine. Does this mean I already need to replace the tubes? Just the preamp, the poweramp, or both? Or is it likely something entirely different? I've been searching extensively and haven't come across this specific issue, so posing the question myself. Appreciate any assistance. Thanks.