500W Linear Amplifiers:
Elecraft KPA500 - $2299
Ameritron ALS-500M - $849 (does not include 6m)
1500W Linear Amplifiers
ACOM 1500 - $3989
SPE Expert 1.3K-FA - $5000
Alpha 9500 - $7995
Alpha 8410 - $5995
The Elecraft KPA500 is simply the standard bearer for 500W amplifiers and its mettle has been proven in many DXpeditions in far away places. The Alphas are the standard in the upper end. The price difference ranges from $1690 all the way up to a whopping $7146.
The question is - is all the price difference worth the extra 1000 watts. Lets look at what you are really buying in terms of what you can hear. 10*log(1500/500) = 4.77 dB or 8/10ths of an S unit.
Let's not confuse the issue between twice the power and twice the volume. The latter is quite subjective
3 dB = twice the power (Power respectively intensity - mostly calculated)
6 dB = quadruple the power, twice the amplitude (Voltage respectively sound pressure -
10 dB = "twice" the perceived volume (Loudness nearly sensed psychoacoustics)
I would say that when referring to perception of something being twice as loud to the human ear that 10db is about right. Try it out for yourself with a vocal on a stereo receiver/amplifier that controls volune in dB since our ears are most sensitive to the human voice.
Here are instrumental recordings for you to listen for yourself: Look for the 2:27 PM post.
Recordings - Reference, 1dB, 10dB and 2dB
You have to go somewhere in the middle of the thread to hear the recordings. I recommend using headphones.
So an 800W amplifier will correspond to X when a 500 Watt amplifier is Z - 2 dB difference. I can hardly tell the difference between Z and X. So a 1500W amplifier is slightly louder than X, when Z is 500 Watts - but how much louder? They say that 3 dB is the threshold of being able to perceive an increase in loudness. By the way Q is like 100 watts when X is 1000W. Also Z is like 800 Watts when Q is 100 Watts. There is a good amount of perceived difference between 100 Watts and 500 watts but the incremental improvement above 500W to 1500W is indeed small. Remember that dB's are all relative measures.
I dare say that between $1700 to $7000 you could have spent on the legal limit amplifiers could be better invested somewhere else - and nowhere to that price tag. Because you can't work them if you can't hear em no matter how much power you are putting out, I am of the opinion that you are better off spending a little bit of money in filters and equalizers if not the receiver itself. Assuming you don't want to invest in a new and expensive high end receiver, my suggestion is to port the audio into an amplified equalizer / filter. The ear is most sensitive to the midrange of the frequency spectrum based on the work of Fletcher and Munson. Filter out 600 Hz and below and above 6000 Hz and you filter out any associated noise as well - so all you have is the frequency content that the ears are most sensitive to. This equipment can be purchased for about $200-$300.
Another worthwhile investment is that of receive antenna systems. But now we are talking between another $200-$1000 depending on whether you have land or not. If you have plenty of land, beverage systems are quite inexpensive. If you have no land, then receiving loops like the K9AY will do the trick. I have been successful in employing transmitting full wave loops which are very quiet in terms of noise. I have used the vertically polarized 30m delta loop during the 13Colonies special event while operating as K2L with good results. Full wave loops are very cheap to make - really just a hunk of wire with a 4:1 balun to match the rigs impedance. In fact I worked Heard Island VK0EK, Juan de Nova FT4JA, RI1FJ Franz Josef Land and several other difficult expeditions using such an antenna. The 30m delta loop nearly outperformed the Hexbeam during the 13 Colonies special event not because the loop is louder but because the atmospheric noise is so low that the signal to noise ratio so high that even the weaker signal of the loop is quite intelligible.
Fletcher-Munson Curves Below
So there you go, get an inexpensive tube amplifier like a Clipperton L or Drake L4B which will easily do 800 Watts (for $7000 dollars less) and you are almost on par with a legal limit amplifier on the other end. Better yet run it at 500W to preserve your tube finals. With your listening system in place - you can hear them when they answer "Your Call 5NN". Without the latter, no matter how loud you are on the other end - it does not matter, you won't make the QSO.