Your discone is simply another random antenna - it's a bit of wire in the sky, but with HF there are other factors that appear to make things different. When you change aerials and notice the background noise go up and down, remember the physics. if the background noise goes up, you are receiving more signal. Noise on your frequency means the aerial is capturing more RF. The snag sometimes is that the RF you are capturing is coming from a different direction to the signal you would like to hear. If you live near a noise source - like a busy city, full of EM noise, then your weak signals need to get above it. Some of those shortwave broadcasts use mega power, so leap out above the rest. Moving to the discone (a worse HF aerial) means you still have plenty of the broadcast station, but less noise. Subjectively it sounds better. It's just not going to be able to 'collect' the weak signals. When you use an ATU, the background noise leaps up when it gets to resonance. The actual capture ability of the aerial hasn't changed at all - just the matching to the receiver. I've been doing some listening trying to compare different aerials - with an aerial switcher and using SDR. These really are pretty rotten radios with loads of spurious signals that they generate internally, plus my computers mot lots of RF rubbish which then gets picked up by the SDR and displayed on the screen. It doesn't really mattter I suppose, but I noticed that switching to the airband VHF aerial removed lots of these computer generated noises. Great aerial, but then I noticed the weak signals perfectly listenable to on the other aerials, with all the sproggies was missing. A much quieter aerial, interference wise, but actually just not a good aerial anywhere other than air band. I think this is what you are finding with the discone on HF. My discone is pretty rubbish on HF - just hopeless at capturing the weak stuff. My ATU lets me tune it for transmit too - but the long wire I used to have up till the tree trimming people came, was much better - it picked up much more noise! Which is of course, just unwanted signals.