You're right, the way you are currently training it would be impossible for you to throw on some big muscle mass. I lift for about 90 minutes five days a week. During those 90 minutes I may only do 16-18 sets with no more than six reps per set. I can lift a hell of a lot of weight, but I would probably fall over dead if had to run two or three miles. By contrast, in high school I held a state record in distance running and I was always at the top in 400m swimming competition. It all boils down to what you are conditioning your body for. You are correct that genetics play a big roll in an individuals ability to build muscle mass, as does age. During the first two or three years that I started lifting (for mass), I put on about 40 lb., most of it was muscle. Since then I'm lucky to gain 4 solid pounds per year. I did this without taking any drugs or supplements. By contrast a thirty year old friend started lifting with me a couple years ago and I'd say he's up about 20 lb. He is training at my intensity under my same schedule. I really doubt he will ever see the gains I did and my guess is it's mainly do to genetics.
Can someone get big w/o taking drugs? Absolutely! A friend of mine is incredibly large (6' 1" and 275 lb.), all drug free. Granted he's been lifting for about 16 years, but he's clean. I use to get really upset when people would say that I'm using steroids. It took A LOT of hard work to get to where I am now and it pissed me off that people couldn't respect that. Now I look at it as a compliment. People just automatically assume that if a person is very muscular, he must be using steroids. Up a certain point that's just not always the case. Yes, top level bodybuilders are taking huge quantities of everything that can dream up. There is a huge difference in body fat between Paul Sr. (25+% ?) and some pro bodybuilder (<5%), even though they may be nearly identical in height and weight! I've done a lot of playing around with my body fat levels. It's amazing how much strength and muscle mass you loss when you drop body fat. The more fat you have the easier it is to build muscle mass. Look at Paul's face and chin. He carries a lot of fat all over. The thing that Paul Sr. doesn't have is a huge spare tire around his waist... but you sure as hell aren't ever going to see his abs either! Everyone carries their body fat differently, in Paul Sr. case it's more flattering that most men with the same amount of body fat. I don't think he's on the juice. He looks like he's played with some GH in his day, and probably been pounding the weights pretty hard for many years.
Supplements? I have tried most of the OTC stuff you mentioned (protein powder, creatine, andro, hgh...) and I've found most of it to be a big waste of money. Of all of them, I've only noticed results taking creatine. This stuff does make a SLIGHT difference, but I've stopped taking it for health concerns. Like steroids, the creatine seems to make your body carry excess water but to a lesser extent. It does have some short term side effects and I'd bet long term effects will start popping up within the near future. I'll be amazed if it's still on the market ten years from now. Anyway, most of the supplements on the market are snake oil. You'd never know it by the $$$ spent on these products each year, but just about all of it is useless. People love to take shortcuts. They think if they spend $200 dollars a month on supplements, they too look like the guy on the bottle. Wrong. The only way to put on real size is to train hard (and properly) and let your body heal by resting. It takes an incredible amount of hard work, time, and dedication. Even these guys that look like freaks just don't get the credit they deserve. I don't care what drugs they're taking, it takes some serious work and discipline to get that big.
Just my 2 cents.