What is your source? The amp needs 2.5 volts input for full power so if your source can’t do that than a preamp would help. Even if you don’t need the extra amplification my experience is a quality tube preamp will add a little secret sauce to the sound that can make it sound louder even if the amp isn’t putting out any more watts. Same with the KT88, I don’t think you’ll get any more power, but it might sound like you are.

How old are your current power tubes? It’s also possible they are getting weak. Only way to know for sure is to try a new pair.

I use a ZBIT for a little extra added umph from my Oppo BD105.  Also using 6n2p preamp tubes gives the unit a little more gain, but at the expense of headroom...gain to distortion.  

I also use KT88s on my Rachael.  I noticed a better bottom end and nice treble detail.  

For my TT, I chose to take the output of my ZP3 into a NAD 320BEE, and made that a preamp as I could use the remote.  The Oppo also allows me to use it's remote as a volume control.  
I've set my Rachael to max volume, and away we go to listening Nirvana.