I have an Elekit 8900 I use with 93db speakers NX Otica with the OB subs. I started out with 300b tubes and had plenty of volume and recently decided to try 3.5 watt 2a3 tubes. Surprisingly, it sounds better to my ear even with what most would consider an under powered amp. I don’t listen loud anymore, but it easily plays loud enough for my normal listening levels. I listen sometimes in the mid 70DBA range with measured peaks in the mid 90db range and it sounds unrestrained and enjoyable to me.
I am using a high pass filter so the signal to the amp starts rolling off at 80hz, I have plenty of punchy and defined bass and even watch movies with this setup and it never sounds strained, keeping in mind I don’t listen real loud anymore. I’m not sure how much head room I save from this setup, but I think letting subs handle some of the bass helps when using low powered amps that otherwise could sound more strained or run out of juice before reaching satisfying listening levels.