...The v3 Rumble get's such good reviews but cost so much less than other brands...it leaves me wondering do the heads that cost nearly twice what the rumble does actually sound that much better?
Audio engineers are clever, resourceful folk. Given a little prior art and R&D time, a competent engineer can wrangle just about any sound he or she can imagine out of a pile of components. So as far as sound goes -- electronically, anyway -- the engineer's cleverness and upper management's requirements determine what the product will sound like. Generally speaking, the electronics sections of an amp or combo are the cheapest parts and most of those components are commodities.
For the mechanical side of things, starting with the amp, all the potentiometers, jacks, switches, etc. have a definite hierarchy of quality. CTS pots, Switchcraft jacks, etc. are higher quality than the no-name Chinese equivalents. And they cost more.
And the cabinets have a lot of opportunity for cost savings. The kind of wood that is used, the thickness of the Tolex or whatever off-brand covering can be sourced, the joinery, etc. all influence sound and the final manufactured cost.
Finally, the speaker has a huge influence on the final sound and the manufacturing cost of the product.
So how does Fender price their Rumble line so competitively?
The amplifier sections make the sound that Fender management and engineers want. Cost: no more and no less than pretty much any other solid state amps.
The cabinets are made from locally-sourced woods and the joinery is "basic" (white glue, cleats, and staples.) Money saved. Downside: very slightly less robust cabinetry.
Hardware: big cost savings here. The pots, jacks, and switches are all downgraded from boutique products. For those of us in the USA, it's the "Radio Shack" formula and it's been in place for decades. Squier instruments use the same cost-cutting measures. It's no coincidence that many Rumble problems involve scratchy pots or intermittent jacks.
Speakers: The Rumble line uses great speakers for the price point. Without looking at Fender's product cost analysis, I can't be sure, but from what I hear and see, the speakers are the finest parts of these combos.
Finally, we throw all these pieces together using streamlined procedures and low-cost labor. This is another area where Rumbles have occasional small problems. Loose screws and cold solder joints seem to crop up on this board from time-to-time. Some cabinets have some acoustic batting inside, others do not, and some members like the sound they get by adding some acoustic batting or foam. Either way, it's a cheap and easy modification.
That should pretty well explain the cost differences between Fender's low-cost (Rumble) line versus their higher-cost (Bassman) line and other manufacturers' boutique lines. I just winged this so hopefully I didn't make any glaring errors or omissions.