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If I may add, from experience, following and including Romy's post above:

1) In cases of established incompatibility with a average power SET, more output energy (55W), will not completely clear the above 2) and esp. make 3) less "muddy" or "sticky" if I understand it correctly. It will remain flat & wet and will not go significantly louder either -- in case you wondered. There is still no Compatibility.

2) Average sensitivity or higher, speakers may still not offer compatibility -- check following 1) & 2) above. Already 1) should give immediate indications and you won't need to consider 2 or 3. If inconclusive, just change the volume level (and/or move closer to the speakers if necessary -- which probably means the spkrs are too insensitive, see below my "3").

3) Average or higher output SET ("+W"). 
One feels the speaker may be compatible but the results from the MF & upper MF check are strange. Maybe a higher watt ("+W")capability can deliver the goods?
In this case, take note of the upper midbass with the inconclusive SET. Try the +W. There may be compatibility if lowMF is now improved. The MF checks 1) & 2) above should fall into place automatically and consistently. The amp-spkr combo is OK. If the lower MF to MF region is NOT improved, then there is no chance of compatibility (i.e. don't spend too much time checking it out) despite any other improvements, and one is best off buying the cheapest SET as a fashion statement.

4) Another quick check -- maybe 1b': "The camouflaged midbass"
If you hear a nice, euphonic, sweet, honey, whatever, midrange sound with lots of nice "midbass" information right from the start, you may immediately conclude that 1) is satisfied. Forget it. Most MF is recessed & upper MF non existant. You don't need to consider 1-3 above.


Indirectly on topic
5) An indicator of SET sonic performance can often be had by using a planar/ribbon etc speaker. Take note of the sound as in MF checks 1 & 3, above, and disregard 2). (I don't know why, admittedly, but it works in very many cases.)
BTW, I don't like planars, nor ribbons, so can only recommend these as a possible convenient tool (since many people seem to drive such spkrs with SET, so they are available for testing).

BTW, Eduardo de Lima has some thoughts on amp-spkr interfacing  here seen as an equivalent circuit (the Glass Audio articles).

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