I had an HM2 waaaaay back when, and I loved that thing! Might be the simplest way to go actually. I bet just having some stomps to choose from through the Cornford might truly kick some can without taking too much of a toll on the bank balance… I personally hate the Metal Zone unless it’s through a loop.
As far as amps, though - have you considered a Soldano? A few popular amp circuits out there based on Soldano Slo100 - 5150, JCM 2000, JVM and Dual Rectifier for sure I think…
You could try the Bluguitar Amp 1 - the ‘iridium’ version is supposed to sound more modern. It has 4 channels (including clean!), onboard reverb, gate and boost. Its similar in format to the victory amp pedals in that it is a fully fledged amplifier - poweramp and all!
I’d personally recommend the 50-watt EVH (original or EL34 version)
The lunchbox amps are great, but the low end and overall tone is lacking due to power amp section. The EVH amps have a great master volume, so the extra wattage won’t hurt.
Failing that, the PRS MT15 is a solid contender with 6l6’s.
The ENGL savage 120 is a classic metal amp, doesn’t need a boost. Nice and almost ‘pre-mixed’ type tone right out the gate. ENGL are indeed more refined and smoother (like they’ve got a built in HP and LP) which I think appeals to a lot of people.
For metal on a budget, it’s REALLY hard to beat a 5150 of some form. The original and 5150 II can be picked up used for fairly cheap, and throwing everything on 6 is instant melo-death. I don’t love the tone either, but I’m the first to admit I’m not THAT metal, and anyone who is has probably spent some playtime in one. And, even today, they’re cheap. I’ve heard of people picking them up for $500 often enough.
Aside from that, if you spend a little more some flavor of Rectifier is probably a good option - I don’t know where pricing is on the Multi-watt Duals now that the 90s reissue is out but if your budget is more like $1500-2000, then a Rectifier and a Tube Screamer will also make a lot of metalheads feel at home.
Those are really the two “core” places I’d start, 5150 or boosted Recto, for the tight, technical, fast tracking low end stuff, where most of the guys coming in your studio will have some level of comfort with the rig. As you go beyond that it really starts to become a matter of taste.
Doesn’t the Cornford have an effects loop? KSR Ceres (modern metal preamp pedal) in the loop, plug your guitar directly into the Ceres, bypassing the Cornford’s preamp. $400, tight modern high gain, voila. I would pit this setup against any lunchbox amp.
Or, cheaper option, and maybe you already have these on hand, Tube Screamer into the front end of the Cornford, Drive knob all the way down, Level knob to taste, then place an EQ in the loop for added flexibility. Only ~$200, you might be surprised.
Source: I’ve been playing down tuned extreme metal for about 30 years. Obviously you could get more rig for more money but it’s probably not necessary for your purpose.
In my experience and subjective view, there is definitely a “too modern” sound from some metal amps, for certain styles/riffs/situations. I’m not sure that you can reduce modern-sounding-ness from the hyper-modern sounding amps, but it is my understanding that you can increase the modern-sounding-ness of the fairly-modern sounding ones…
While I don’t know what else you have in stock, why not consider the Mesa Mini Rectifier or Badlander lunchbox amps and offer guests a tube screamer and/or MXR EQ pedal to sculpt towards extra-modern, if desired? Would it not be a truly rare artist, who couldn’t make that work?