I was under the impression that the OP was looking for a low/no $ option.
but Johnny is on the money - if the quality of the DI signal is important then a good DI will make a significant difference.
For DI's, this is still my favorite.
http://www.countryman.com/type-85-direct-box/While I haven't compared it directly against radial engineering's boxes (which I've heard nice things about) my experience / preference has been to always go with an active DI (Radial Engineering does have an active DI version).
The Countryman can be picked up used on Ebay or Reverb at a decent price.
On the other hand the 'quality' of the DI signal may not be significant factor when using the Toontrack metal packs. Depends on how it uses the original signal and other factors.
With Amp modelers like Line6 having a better quality DI signal doesn't usually correlate to a better result. I switched from using a Line6 DI because I thought the output was too low and lacking quality but found that a hotter cleaner signal made the PODFarm plugin sound terribile - it needs a lower input to get the best results.
Also the first piece of advice many pros give metal heads or high gain amp fans is to use a band pass filter to roll off the highs & low end off your guitar signal before passing the signal to your virtual amps.
So just saying that you could end up spending time/money pursuing a pristine DI signal and then have to throw away the gains to get your desired amp sound.
Maybe a Toontrack Metal user can comment on what the plug prefers for input.