Scott, You point out an interesting difference between HF and UHF...
<quoted> The preamp was a concern for me in that since I've been working HF, I've found that I receive much clearer with the preamp disabled and the attenuator enabled and just pushing up the AF gain. I realize this is due to noise in the area and the fact that the IC-7000 preamp is a broadband preamp, and generally will just increase the noise floor and lower the sensitivity of the frequencies I'm actually interested in. <end quoted>
The main difference is the noise of the exterior environment. Without going into the gory mathematical details, essentially the noise figure of the exterior environment is on the order of 40-60dB on 40m, decreasing to roughly 5-10dB on 10m. Most radios have RX noise figures on the order of ~10dB. Basically the radio only needs to be a bit more sensitive than the external environment--which is why attenuators are more appropriate for 40m, and sometimes (but not always) a preamp becomes helpful up on 10m.
At 70cm the external noise is on the order of a couple of dB...but the coax and radio push the total up to ~12dB...so a low-noise amplifier at the antenna can make a LOT of difference. I once made a calibrated off-air measurement of the sensitivity improvement rendered by an ARR GaAsFET preamp on an M2 eggbeater (another omni antenna) and found 8-10dB improvement.
Scott Townley NX7U Gilbert, AZ DM43di http://members.cox.net/nx7u