If you want to know whether that particular Adcom is better or worse than that particular Onkyo, then you can make that determination as you have both units. It really doesn't matter what anyone else thinks.
However, if you want to know as a generality whether a receiver is better than tuner/preamp, then I think that the answer is not absolute. It depends on the particular components. Usually, the receiver is not as good, but it doesn't have to be that way. Receivers are typically built for the mass market, or the entry level audiophile market. Price is an important consideration at this level, so the manufacturers shoehorn in as much as they can so the consumer gets more for their money, so to speak. And they use cheap parts. And they have a big honking transformer that is unshielded and emitting an electrical field that is interfering with the sensitive electronic components. And there's a single power supply for all the channels.
Now if a manufacturer wants to, they could (and some do)make a receiver with separate power supplies for each channel, everthing is shielded, the preamp section is totally isolated from interference, etc., etc. In other words, it's just as good as "separates". But the price will be a lot higher. However, the market for higher priced products, i.e. the audiophile, typically doesn't want a receiver. They want separates so they can upgrade and mix and match more easily. As a result, there are not whole lot of really, really good receivers that would best good separates. It would be better than poor separates though. Being a separate doesn't mean that corners can't be cut to save money.
So, in the end it still comes down to the particular units being considered. The receiver may be better or it may not be. You decide which you prefer. Myself, I think the Adcom should be a little better than the Onkyo's preamp section. But I don't know for sure since I haven't heard them. You have them so you tell me.
A couple other points.
I don't think the place of manufacture is important. It's the ratio of quality level to price point that's important. If it's a well made product, I couldn't care less whether it was made by gnomes living beneath the mountains of Switzerland. Your "Made in the USA" product could quite possibly contain electronic chips, or other parts, made elsewhere.
There's a lot of variability in how specs are measured. It's difficult to compare them across manufacturers. Use specs as a guide but trust your ears for the final decision.
However, if you want to know as a generality whether a receiver is better than tuner/preamp, then I think that the answer is not absolute. It depends on the particular components. Usually, the receiver is not as good, but it doesn't have to be that way. Receivers are typically built for the mass market, or the entry level audiophile market. Price is an important consideration at this level, so the manufacturers shoehorn in as much as they can so the consumer gets more for their money, so to speak. And they use cheap parts. And they have a big honking transformer that is unshielded and emitting an electrical field that is interfering with the sensitive electronic components. And there's a single power supply for all the channels.
Now if a manufacturer wants to, they could (and some do)make a receiver with separate power supplies for each channel, everthing is shielded, the preamp section is totally isolated from interference, etc., etc. In other words, it's just as good as "separates". But the price will be a lot higher. However, the market for higher priced products, i.e. the audiophile, typically doesn't want a receiver. They want separates so they can upgrade and mix and match more easily. As a result, there are not whole lot of really, really good receivers that would best good separates. It would be better than poor separates though. Being a separate doesn't mean that corners can't be cut to save money.
So, in the end it still comes down to the particular units being considered. The receiver may be better or it may not be. You decide which you prefer. Myself, I think the Adcom should be a little better than the Onkyo's preamp section. But I don't know for sure since I haven't heard them. You have them so you tell me.
A couple other points.
I don't think the place of manufacture is important. It's the ratio of quality level to price point that's important. If it's a well made product, I couldn't care less whether it was made by gnomes living beneath the mountains of Switzerland. Your "Made in the USA" product could quite possibly contain electronic chips, or other parts, made elsewhere.
There's a lot of variability in how specs are measured. It's difficult to compare them across manufacturers. Use specs as a guide but trust your ears for the final decision.