Saying a power tube type is "loose" is like saying a pickup is "loose."
It's not.
No power tube is inherently "loose," just like no pickup is inherently "loose." The word "loose" in the context of guitar, specifically, is a word describing what happens when low frequency content becomes distorted to the point that it interferes with and muddies up the frequency content above it.
A passive pickup always outputs a completely clean signal. A preamp can become oversaturated and produce a loose and muddy sound if enough gain is dialed in and the preamp doesn't have sufficient low end filtering at the amp's input, but that's not the pickup being loose, but rather the way the preamp processes and distorts the low end from the pickup causing the looseness.
All power tube types distort in roughly the same way into a resistive load. If any given power section of any given power tube type is distorting in a way that sounds "loose" and muddy while the preamp is clean when the amp is turned down, it's due to too much total output volume and relative low end from the preamp being sent to the poweramp. But any tube type is going to behave in a similar way, not just KT66's.
If you turn down your amp's output volume to something like TV volume level and you're still getting a loose and muddy signal, then it's your preamp producing the looseness and the poweramp is simply cleanly reproducing the preamp's distorted looseness at all volumes, just like how listening to a recording of a distorted guitar through a high end stereo at low volume won't clean up the distorted guitar.
If your preamp sounds tight and you turn up your amp's output volume and it distorts and gets loose and muddy before you reach your desired volume, then your power section simply isn't powerful enough overall, but again that's true regardless of power tube type. In that case, you could build a 150w KT66 power section using six KT66 tubes and if you kept the same output volume, the looseness would clear right up even though you're still using KT66 tubes.
