I don't know what kind of amp you are looking at, but any decent solid state amp should work OK. I wouldn't fool with the speakers themselves, but if you need to make them an easier load for your amp, an external Autoformer may be just what you are looking for. I know that Speltz (My spelling may be off - The same guy that makes the Anti Cables) makes them and they're not too expensive. I don't know if you can increase the sensitivity but you can probably turn it into an 8 ohm load. It would be worth looking into.
This probably a really silly question...
I have a pair of Sound Dynamics 300ti that Harry Pearson liked back in the late 90s. The 300ti is an 89db speaker, 4ohm impedance. My question: is there a way to modify to higher efficiency and bring impedance up to eight ohm? This speaker has a 8" woofer, 5" midrange and 1" or less beryllium tweeter.