Amps or Speakers which one to upgrade next

Options

1 Change your speakers. It won't be easy because no loudspeaker can be much of an improvement over what you have. They can be different though.

2 You already have a great amp. Again there's nothing much better out there. Just different, you probably don't want to go back to solid state.

3 Its certainly worth experimenting with sitting/isolation as both your speakers and amp are susceptible to resonance effect.

4 Check out good quality recordings/masterings. Your system deserves it.

When you're already near the summit, there's not much left to go. A second pair of ears is also essential at this rarefied level to prevent hallucinations.

A good recalibration trick is to listen to a extreme low budget system for a day or so. You'll soon fall in love with your current setup again.

many hold to the idea that front end is most important as in "garbage in, garbage out"

I disagree. The most colored component is the speaker. It is the starting point for upgrades after acquiring a competent set of components before it.

Proof? take some very high end speakers and power them with a wide variety of amps and front end components. Even though the sound will change the overall characteristics will remain the same. Then audition a wide variety of speakers and the sound changes drastically. That means you should find speakers you like and build a system around them. If you pair a $100K speaker with a $1000 amp it will be far superior to a $100K amp driving a $1000 speaker (assuming the amp is capable of driving the speakers)

as far as investing in music, unless you are vinyl only, with the proliferation of high quality streaming sources these days music is very cheap

Before you spend a penny on new equipment or room treatments get this:
https://www.musicdirect.com/accessories/IsoTek---Ultimate-System-Set-Up-Disc-CD
No affiliation.
I was really surprised what a difference repositioning my speakers made ad the unconventional position that sounds best. I would never have them set this way if I didn't hear the difference using this CD. But it may be mostly because I have a very asymmetrical room. Still the difference is at least on the order of a speaker upgrade or more for me.

unless you increase your budget for much better speakers do what simao said.........more music !

Hi Tomstruck,

I would agree with those who are advising you to invest in cabling.Your $7k investmentment will go alot further by purchasing  a top rated power cable. It will provide you with the some of the bass and clarity that you are looking for.

Also consider switching out your speaker and interconnect cables. Another consideration is to roll out your driver tubes (12AU&X7) for some NOS. Rogue is really friendly to Tweaks and responds well to power.

Making these changes will maximize the performance of your existing system. Check these sites out for ideas. An investment into any one of these kinds of cables would shurley be your last. 

Finally, after these upgrades if you still require more bass,  you could  add a subwoofer, as the cronus allows for it. I would not go beyond a 12 inch driver. I think REL. Older B1 would work really well with the Vandersteens.  

In my humble opinion - Best bang for buck scenario, Good Luck

Thales - use for speaker cables and  interconnects 
www.tonarm.ch

Stage 3 Concepts - Best for power chords
www.stage3concepts.com

HB cable design, Best for power Distributer if needed.
www.hb-cable-design.com

BRENT JESSEE RECORDING & SUPPLY, INC - great NOS tubes
http://www.audiotubes.com/