Hey Stefan,
I just recently finished having to run brand new wire, as I had none previously. I was Completely finished when an inspector drove by, came over and started pointing out areas that did not meet code, so I had to completely redo EVERYTHING..trust me I can appreciate your circumstances.
You will need to check your local codes for exterior wiring, as for 1, you have to typically have a GFI, which the #8 may or may not be running to. Most Spa boxes today have the GFI built in, and already have a 40-50amp installed. If the manual is asking for #6, its because that is the amperage the hot tub requires to run at its optimum load, not only from a functional standpoint but from a safety standpoint, you will need to go with #6 guage if that is the recomendation, and upgrade your spa box to a 50amp breaker, as well as throw in a 50 amp on your circuit breaker. (Honestly, if you go to one of the box stores, the amps are around 10$ a piece, the conduit is dirt cheap, and the wire, is cheaper if you buy #6 awg (#6 in red, white, and black, but #10 in copper). I would strip the copper, recycle it, then purchase the #6 individaul green ground, and run them all through 1 inch conduit. That is the standard outdoor requirement for spas today. All 4 wires, being at a minimum #6 guage.
You can do it yourself, but you will typically will have to get a permit from the city or L and I. (typically like 4o-50$, then have it inspected when its all done for your own safety).
I did all the wiring myself, and with everything I spent:
Spa Box - $100
Conduit - $20
Wire - $50 (from circuit breaker to spa box)
Wire - $120 (from spa box to hot tub. I bought lots of extra to be safe...had almost 10 out of 45 feet left over)
Conduit Bond - $4.99
Permit - $46.00
But now I have the amp/watts required for my new tub to run at its best, I'm up to code, and I'm safe.
Any questions feel free to msg me on it. I had to go through alll the 'riggemoro' so its all very fresh...
Good Luck!