For all I know, some people like the OP’s guitarist can in fact detect an instant, non-adjustable deficiency in anything lacking tubes, but I have my doubts.
There are soooo many parameters you can adjust in something like a Helix that I suspect quick replies of “just get a tube amp” to be knee-jerk reactions. Confirmation bias. Anti-modeler/pro-tube amp confirmation bias does have a characteristic ring to it, and I hear it in this thread.
Maybe more to the point, the OP seems to have space, budget and transport limitations that strongly favor working with what he/she already has. Since that listed gear includes a hugely flexible front end, a powerful back end, and limited cab size/weight/displacement, maybe the focus should be more on “how can I approach a sound like _____ in a _____ playing situation with a Helix, 1,000-watt Peavey head and 2 x 10
Cab?”
To answer something like that, the focus probably needs to shift off of “achieving a valve tone with solid-state gear,” because what seems to be getting lumped in with a preconception of “valve tone” is physical cab/driver displacement, dirt, compression and EQ. With a Helix, or any number of other modelers, attaining a whole range of dirt, compression and EQ possibilities should be do-able with tweaking, and listening in a given band mix. Screw how valve-y it sounds or not...how does it fit the mix?
The cab will still be a limitation, but if that’s all that’s available because you drive a Miata or have to carry it up and down 9 flights of stairs and the service elevator is broken...what can you do with what you’ve got?
Probably need more info on how loud it needs to be, what it’s supposed to keep up with in performance, whether it’s getting some PA assistance or is the sole source of bass reproduction. That will help determine how much can be expected and what limitations need to be accepted.