Tornado and Thunder Power Cables

The Rotels that you own are 'beasts' of an amp. Rotel engineers will have designed a wonderful and suitable power supply inside them, one that does all the mains power filtering needed. An (overly) expensive power cable won't / shouldn't be able to add much to that. If Rotel didn't do a good job, you wouldn't have bought them in the first place.

My guess is you could spend your money on other nice or fun things because you're probably not going to hear a difference, besides maybe the psychological effect that your brain wants to hear a difference after you made a serious investment.

If to you such power cables fall in the category of nice / fun things to have then to answer your question: your 2x350 Watts amp, delivered via 110V requires 7A, or or 3A if you're on 240V. The cable should be rated for those 7 Amperes.

Has anyone used Cullen Power Cables. 

It’s just that home theater receivers aren’t exactly known for being low noise / distortion enough for some to be considered ‘hi-fi’ at least for clean 2ch music listening (but Marantz is def good consumer-level stuff)… So if this is about lowering noise or cleaning up distortions, you may get better results going with lower distortion ICs first. This will also help aftermarket power cords have a bigger perceived difference. 
 But even better money spent is the recommendation to get a good hifi 2ch preamp, with HT bypass function, for 2ch music listening. Then you’ll be stepping into the lower noise / cleaner signal realm and ICs and PCs will make a greater difference. 
 Just as an example: Not affiliated but I recently tried some Hapa Audio copper cables and was amazed at how clean of a sound and less distorted they were over my other similarly priced IC options.

The AQ thunders are great. I have one on my regenerator and it makes a huge difference. The ones above who think that power cables don't make a difference when you have a "good" power supply, is severely mistaken and hasn't bothered to attempt it on his equipment. Cables definitely impact sound. 

I will agree with the others to where your cables are outclassing your amp. You might get more of a benefit putting that +$4k into an amp. 

Rotel amps are definitely not one that is recommended for 2 channel listening

Preamp- Marantz AV8805: $4500
Power amp(2 channel)- Rotel RB1590: $3700
Power amp(5 channel)- Rotel RMB1585: $3700

I’m a little bit confused ? What am I missing here? The OP got $11,900 worth of gear and his gear is NOT recommended for 2 channel listening!? We all should be so lucky!

I say go for exactly what you want - the Audio Quest Thunder at around $900 each is perfectly reasonably priced - for YOUR budget. And do NOT believe the naysayers that say there will be no improvement in sound quality. You will get quieter and blacker backgrounds and better and less fatiguing sound coming from your speakers.

Also, I think it maybe a good idea using all Audio Quest PC’s as they will have the same sound signature and blend in well with each other, especially if you are planning to get a Audio Quest conditioner at a later date.

And one more thing, don’t stop with other upgrades and tweaks. Damping, IC's, speaker wire and fuses to name a few.

Sheesh! What a tough crowd!