Mwelliott,
There are many issues which need to be addressed before coming to a decision of buying a new amplifier.
Your system is used for both music and theater. Is it set up for theater or is it set up for music? The 2 set ups are usually very different - I've never experienced a system that was able to be optimized for both.
While a sound system can be only as strong as the weakest component - this assumes the components are set up to perform their best.
I've personally visited many audiophiles who invested more than $40k in their system and helped them re-set and adjust their system to improve it's performance dramatically. And I'm no one special.
Also, no mention of the other components in your system were made. Speakers, cables, power conditioners, etc. all make a significant difference.
Let us know what your system is set up for - theater or music, what your speakers and cables are, if you're using a power conditioner - and any significant accessories you may be using.
The ARC Ref 3 is a great pre-amplifier. While I'm not a Krell fan, Dan D started the Krell company by building amplifiers (just like he started his current company with his Momentum amplifiers) - so your Showcase amplifier may not be the weak link. Your set up, speakers, wires, power conditioner may offer more significant improvements.
And, after considering (and making changes / adjustments to) these points - if you want to upgrade your amplifier - you'll have a better understanding of how things are working in your system.
Only good wishes to you.
Life is too short not to listen to music that stirs our senses and moves our souls, TJF