1st, I'm not a "receptor" guy. If receptors down graded to any significant degree we would see HRT doses going up the longer someone was on- haven't seen it (or if you have please send info to my doctor).
Having said that, I think you need to look at why you make the best gains during the 1st 4-6 weeks of a cycle. There are many factors I don't think you are considering.
1. Distance from your genetic potential. This may not be the best name for this but like it is easier to loose the 1st 15 lbs on a diet rather than the last 5, it is also true that it's harder to gain 5 more lbs after putting on 15.
2. The body's ability to adapt to food intake. Now here I think you'll find the main culprit. There are several examples I could give but let's take the most extreme- the BBer who just finished a contest. He's at a very low calorie intake and he's been there for weeks. He's fighting tooth and nail to get that last couple of lbs off. Now he's done. Here comes the food. Can we say anabolic environment? Also the leap in calories just can't be duplicated. Some people end up 1500 calories or less when finishing up their contest diet. Now if you want to bulk immediately following you can triple your calories intake fairly easily. Even the regular Joe who was eating away at 3400 maintenance can add 1200 calories no problem. But how many bulking diets have you seen start at 4600 and end up at 9200 calories?
3. How many bulkers do you see end up as cutters? This, IMO is flawed design but it's considered standard practice to use AS that "harden you up" at the end. Although there maybe nothing exactly wrong with the AS choices, what does this do to your mindset? It tells you it's ok not to put on any/much more weight.
4. Let's face it, the gains at the beginning are water! Muscle weighs as much as fat (but takes up 1/2 the space) so a lb of muscle (correct me if I'm wrong) takes 3500 calories to make. That's above maintenance levels. Do the math. It's hard to put on more than 2-3 lbs of muscle in a week. But we gain a lot more weight than that. Again H2O.
5. The body raises metabolism to account for the additional calories. Just as you diet, your body fights you to keep weight at a certain point when bulking the body will get used to the additional calories and eventually figure away to burn them off. A fat person for example (follow them around it's really true) can eat an amazing amount of calories and not gain a lb. Why, because their body's are used to the calorie intake. Sure they become fatter but the change has to be measured in months. Someone in the 270 range may get to 275 but it may take a year (over 5000 calories a day) to do so.
6. These same rules apply to naturals also. It's just that naturals have to watch the extremes much more closely, but they make there best gains during the 1st 4-6 weeks also.
So I say it's food, not gear. I would like to hear other opinions on this.
BTW: I loved this question!