Identify System Bottleneck - Digital Front End or Preamp

Need some opinions on whether to upgrade my digital front end or preamp first.

Speakers:  Wilson Sabrina
Amp:  VTL ST-150
Preamp:  Hovland HP-100
Digital Front:  Mac Mini w/ Audirvana3+ & Ayre QB-9
Cables:  Cardas GR

I'm trying to decide between upgrading the Hovland HP-100 to VTL TL2.5i, or upgrading the digital front end to a Mytek Brooklyn Bridge.  The Hovland HP-100 has been updated (twice) to the latest production specifications right before Hovland went out of business.  I've also recently retubed with super low noise pre-war yugoslavian 12ax7s and super low noise telefunken 12at7s from RAM-labs.  My concern with upgrading the preamp:  Will the VTL tl2.5i actually surpass the Hovland's performance or will I actually loose sound quality?

Regarding upgrading the digital front end:  I can only see benefits, but not sure if this is the bottleneck, or the preamp.

Room is highly treated so there is absolutely no need to address room treatment at this point.
I would install audio grade fuses and wall outlets if not yet done so.
David Pritchard
Negative, give me a real upgrade plan. Not hocus-pocus...

Let me restate the ask, again:

"Need some opinions on whether to upgrade my digital front end or preamp first"
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@davidpritchard  gave you good advice, but if it isn't what you want to do, I understand and it's your choice, as is everything in our hobby.

Have you considered trying loaner Cardas Clear Beyond or Clear cables vs your GRs? Might be worth exploring!

I'm not making a statement with respect to the digital front end vs the preamp choice...just that if I were to start the process, I'd begin with upgrading my digital front end, first. And I would explore a number of options (inclusive of the Mytek Brooklyn) that fall within the same budget range.

All the best with your search and choices.
I'll take my question to a more practical site.  Thanks anyway...
Honestly the difference in preamps would not be better the hovland preamp was a reference grade pre.

The ayre was a good dac but is showing its age.

Also a mac mini or laptop is not designed as a music transport so your path to the dac is compromised.

We would recomend a lumin t2 to replace the dac and Mac mini

Dave and Troy
Audio Doctor NJ Lumin dealers
@audiotroy why do you recommend the lumin t2 over the Mytek Brooklyn Bridge?
The Pre-amp is the heart of your system, The Power amps is the muscles. 

Try to see if you can get a MP-3 Atma-sphere pre-amp or a MP-1 (If funds permit)

I would go the opposite way than what audio doctor suggest and focus on getting a better pre-amp to start and than move to the Digital Front End.
Phantom av the Hovland pre was designed as a reference preamp by the guys who bullt the hoviand capacitors.

This was an exceptional preamp.

We sell lumin and Mytek the T2 is just better.

Dave and Troy
Audio Doctor NJ Mytek and Lumin dealers
Speculatively speaking, the answer to your question is the digital front-end, particularly the mac / audirvana, followed by the little Ayre.
Before you change anything, however, I strongly recommend you try a "regenerating /reclocking" device (many of those around, I use this one) —a very good thing to have when you listen to a digital source. Most of them are inexpensive and well worth the outlay. BTW, I'm familiar with your equipment except for the cables; great choices!
Good luck!
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This ones a no-brainer: upgrade the digital front end!
Any turntable will do.

Already been there done that.  Classic signature with Benz Ruby Z!