Has anybody tried using single solid core cables?

I'd like to add my 2 cents to the long list of responses above, pertaining to solid core cables vs. "whatever".  I have a considerable collection of interconnects and speaker cables of all lengths, aquired over the past 50 years. All of them, different designs, based upon bundles of stranded copper and silver plated wires, and almost every concept of insulation, isolation and shielding imaginable.
That said; - - -  along the way, (about 20 years ago) I stumbled into an ad in Stereophile (I think) for a small private company in Florida who was making and marketing Solid core, twisted pair, enameled high purity copper speaker cables, terminated with your choice of quality bananas or spades. The thing that caught my eye was the price. At that point in time I was using 15' speaker cables (due to routing requirements). I called the owner and he said he could make me a pair with bananas and ship them out same day. He even said that I could ship them back for a refund if I didn't like them. And the price was $25.00 for a pair. MY God, how could I not check them out.
When they arrived I just about sh _ _ my pants!! They were so stiff, he might as well used 1/4" copper tubing. Insane ! It never dawned on me that they would be that gnarley. With the way my system was set up at the time there was no way I could use them. Wasn't worth sending them back. So I just coiled them up; - - put them in a box and stored with the rest of my "collection":.
Next encounter ! - - - - about 7 years ago, I ran into a person at an audio show, who said he was a officer of the Chicago Engineering Society. He had in his hand some bold looking (RED) interconnects with VAMPIRE RCA's. We talked about his theory and experiences while developing these cables. Once again they were constructed of heavy gauge enameled solid core OCC high purity copper wire in twisted pair configuration. I thought to myself (Oh no, not this again). So the fellow lived near me and suggested that I take home a pair and try them for a week. Then, either buy the or return them.
O.K., why not!  After a week of some extensive listening and "fiddling", I was so impressed with most of the characteristics of what I was hearing that I handed him $50 and kept them. 
Well, wouldn't you know, over the past 7 years they ended up in storage also. - - - - - until two days ago !!
Very recently I have listened and investigated MORROW cables. A friend bought a pair of phono cables and was amazed. M-M-M-M This is one I passed up. Should I give them a try? O.K. , so I did. The one drawback being that MORROW claimes that some sort of "MAGIC" is supposed to happen after 400 - 600 hours of breakin. Of course they could speed that up with a burn in cycle for a considerable price. I didn't pop for that.
Been using them for a couple of months now, with some mixed results. There were days when they sounded odd; - - almost muddy, but then cleared up. "What's going on here" MORROW said that would happen.
Anyhow to get off the subject of MORROW, - - - - - - -
Three or four days ago, I decided to start cleaning out my collection of audio artifacts. In doing so, I ran across the old RED "ALLIGATOR" cables, as I fondly called them. You know, - - - - I never actually tried them. Well one of my current setups is pretty much modular with components in a open rack. Plenty of room to fool around. So, I disconnected my MORROW'S and plugged in the "RED ALLIGATORS".
Didn't have to concern myself with them lying on carpeting because they are more than stiff enought to suspend themselves. Subsequently, I discovered my RED interconnects, also solid core. You know, I never tried using all solid core through the entire signal path.
To end this long pile of sh _ _ ! After warming up the system for about 1/2 hour, I though I'll try a little serious listening. After about 3 minutes of listening to a VERY familiar CD, I stopped ! I have to try another CD that has always sounded a little harsh, being too closely mic'd. 
All I have to conclude with, is the results were astonishing to say the least. Without any rservation, I was hearing the best overall sound I have ever heard out of my system. ROBUST; VERY DYNAMIC; while at the same time, sounding PURE and TRANSPARENT without any noticible harshness in music passages that had always been there with virtually every combination of wires I have ever tried. Dimensionality of voices and instruments were well placed with appropriate emphasis, in a 3D soundstage, more so that ever heard previously.
You can call what I have said here, a pile of "bull".  Also keep in mind that what I have done here would not be very acctepable to most audiophiles for any number of reasons, but - - - -  If anyone out there is curious enough to locate similar cables and try what I discovered, I believe that you to will be astonished at the totally beautiful sonic presentation. THAT'S IT !!

i second nonoise on mapleshade helix/double helix

outstanding cable

I used to manufacture audiophile cables in the 90s. Music Music Audio Cables. all my cable were multi strand solid core cables. I tried single wire and it either rolled the high end or bottom end depending on diameter. Had to use multiples of various gauge to have it sound right. The only single wire conductor that worked was a rectangular silver plated wire for digital. That said, I think some folk might use silver plated single wire cable. Pick the diameter that gives good base then use silver plate to bring in the highs. That said, Silver plated copper always sounded wrong to me. Exaggerated highs and discontinuity in the sound. As I recall, my interconnects were using something like six 29 gauge oFC copper wire per side and multiple 20 gauge solid core wires for speaker wire. 
I got started making cables after talking to Stan Warren (PS audio) about single core. I used to develop cable by laying multiple bare wire on a piece of masking tape, then placing another piece of masking tape over the top to make a flat "Flypaper" cable. I attached plugs and just played around until it sounded right.
Good connectors are mandatory. I liked connectors without ferrous metal. Silver on brass or Brass with Silver then gold, no nickel. plate to bind the gold. Cardas SLVR and Vampire locking RCA are excellent as well as the ones with a hollow center pin. I also used to favor Wonder Solder for connections. 
So find someone who sells OFC bare wire and have some fun. Don't use these on digital sources or phonos you will just get garbage. Good for CD player to Amp and Pre to amp.
I recently got hold of some cable I made in the 90s and they stand up to some newer model quite well. Obviously technology has improved in 30 year. 
The nice thing about these fly paper interconnects is the dielectric, the paper is as close to an air dielectric as you can get. No Teflon coating or PVC to alter the sound of the wire. 

Best solid core I ever tried is from Acoustic Revive. Sold my Nordost Valhalla and Audioquest and never looked back. Using bare wire, and short lenght. Very analog and organic sound. Still loads of details. Pure music bliss

I’ve tried some VERY cheap solid core telephone wire (0,5mm) for high tones, solid core electric cable (not sure, maybe 2mm) for low tones. OMG the sound was freakin awesome! But of course it’s not a pretty sight all those loose cables. Maybe I’ll try to wind em all together this weekend or something.