This low output seems to impossible if things on the AV receiver are set up properly and the wiring between it and the speakers is proper, and of course, if the speakers have not been damaged. As stated by others, even at medium volumes you should be getting blasted out of your socks.
The K-horns yield very loud output with a low signal reaching it. This may be part of issue. My thinking is that you are getting some acoustic output despite some error(s) in chain feeding them. These errors could give enough electical output to drive the speakers to some listenable level but fails to give the full results.
For example wrong settings may allow some bleed though from a source you have not selected. But there will be enough low level output from the speakers to let you hear.
I'll suggest checking everything in the chain in a somewhat logical manner.
1) Check if things are okay with the AV unit. If it has earphone output, try listening with earphones. It is an easy way of taking a close examination of the functioning of the AV independent of speakers and wiring to speakers You'll have to find a known good source like the internal FM radio (if present) or a CD/DVD player that is hooked up to the correct input, in the back. In the latter case, are the plugs snug? Are they placed into the proper jack?
This may seem obvious but AV receivers have a lot of settings which can fool you In the forest of jacks in the back it is easy to pick the wrong one. Is the printing for the jack below or the jack above? Check your manual. If lost, check the manufacturer's website for a .pdf.
As an example of bleed through: If you have everything wired correctly for CD input, and then select phonograph or DVD input as a source, you might hear the CD, at a very low level -- which the K-Horn will reproduce. (Though the headphone test should show this as abnormally low sound in the headphone.)
2) If when using headphones the sound is reasonable, then check the unit's settings regarding speakers. Some units have settings for A speaker pair output, or B speaker pair output, or Off. Even if the headphones work, the speaker selection switch could be set for Off; which you don't want. See if you can be sure that A is On.
3) Check wiring at the back of the AV to the speakers. There should be something like A speakers that you can identify (per the above you've set these to "on".) Again, with multiple speaker outputs terminals it can be easy to get things wrong. Check the manual for an easy diagram.
You'll see a set of four posts for the main speakers, probably. Look for labels for A speakers. Or main, not surround. That is another bleed though issue.
Take note that the pair of wires going to the left speaker has to be connected to a red (or plus) post and a black (or negative) post for left. Same with the right. speaker. If you connect a speaker pair to the two red you'll get some acoustic output in stereo. But this is a wrong connection arrangement.
4) Further to 3) if you have a mare's nest of wires in the back of the AV, and light is dim, a flashlight can help even if your eyesight is perfect. And are you looking from an angle or looking at things upside down bending over? Is there confusion in what wire goes where? Try making some tabs with masking tape to label wires. Everyone starts off thinking ther're too smart to make an error -- and then find out different. "Nuff said. Smile.
5) The last matter is the connection to the speaker input. Take a close look. If you've been getting good bass, mid, treble; you've probably got this correct.
I don't mean to be condescenting. But it seems that you're dealing with an receiver you have not used in a while and and new speakers you've never used. Errors can creep in. So it is important to grind through things in a logical order.
Let us know what happens. I believe that once you dope out the situation you will be very pleased. Then you can fool around with speaker placement and getting the K-Horns into the corner.