Why there isn’t more good entry level high sensitivity speakers, around £1000?
High sensitivity starts at around 96dB, but that is only really necessary if the amplifier power is below 20W. Such level of sensitivity is only found in very expensive multi-way horns.
Depending on the type of amplifier being used it is also important to have speakers which present an easy load. Topologies with a high output-impedance (usually low or no negative feedback designs) are affected by the speaker load.
As
@rabski wrote above transistor amplifiers, negative feedback and demand for smaller and narrow speakers led to current speakers having low-ish to medium sensitivity and presenting demanding loads.
The small, low-sensitivity, long-excursion midwoofers used in current have a few advantages: they're easier to integrate with tweeters in terms of directivity, extend lower at the bottom end despite the smaller diameter and some designs actually produce less distortion than their lager counterparts.
If you intend to use your amplifier in triode-mode then it makes sense to go for high-ish sensitivity speakers and easier loads.
Because the combined amplifier-speaker frequency response is not easy to antecipate you have no option but to try a few different designs (e.g. those listed by
@Non-Smoking Man ) until you find what you're looking for. The problem is that they're not cheap, even used, or easy to find. Personally I'd avoid any single-driver speakers.