The TS590 shows a 3:1 matching range... 16.6---150 Ohms, that's not 10:1!
That is incorrect and came about from some lazy gimp at Kenwood copy'n'pasting the TS480 info.
I have tested the tuner in the TS590 and it will match 10:1 but as someone has already mentioned you can end up with a 10:1 SWR with massively varying figures of reactance and resistance, some of which the internal ATU can't match.
However here's my 2p worth.
DO NOT USE A SHACK END ANTENNA TUNER. They're what, in my opinion, idiots who know no better use once you're using them for any more than a slight mismatch. Any sensible person uses an antenna coupler. With a shack end ATU even though the rig sees 1:1 the high SWR is still there on the coax between the tuner and the antenna so you will get additional losses because of this. With my inverted L and its unmatched SWR of >85:1 on some bands, those losses can be quite significant and remember those losses are both on TX and RX so not only will you not get out as far, you'll not hear as much either.
An antenna coupler such as the SGC230 which connects directly to the antenna feed point avoids all of that. The SWR the rig sees is what is on the coax.
A long standing amateur at my club disagreed with me quite vehemently. He recently came across the chance to get a MFJ antenna coupler quite cheaply so gave it a go on his inverted V doublet. Not only can he hear stuff that wasn't possible before, he is able to work stations with 100W barefoot that he couldn't do running his linear amp at full tilt.