Recommend me a vintage sounding, but efficient 15inch speaker

[Kulchinski1993]

Hello,
I have an Acoustic 150B with a 402 cabinet (2x15). I'd like to try to get a tad more clean headroom, so I was thinking I would replace the two speakers with more efficient ones.
Can you recommend a really warm, vintage sounding speaker, but still very efficient?

 

[B-string]

Do you know what speakers you have now? That is the only way to know if you can get any improvement from a speaker change.