TT, 12" Tonearm. Who tried and ended up preferring 12" arm?

Elliot, stay away from any tone arm that, "needs repair." A tonearm should never need repair. Any good tonearm is a life long investment. If it needs repair it was abused in some way or it was a bad design.
Any tonearm with an offset angle requires an anti skate devise. Not having one interferes with the cartridges ability to track the right channel (outside) and increase both stylus and groove wear on both sides. You can demonstrate this to yourself with the HI FI News test record. Play the anti skate test and adjust your anti skate as per instructions. Now play the tracking ability test. Gently lift the anti skate device to inactivate it. What happens? You will have an "Oh Crap" moment. The most important function of any tonearm/cartridge combination is to be able to track the record.
The best way to do a dust cover on a turntable that does not have one is to incorporate it with a platform. To be functional a dust cover must be hinged. Trying to place a loose dust cover over a turntable that is playing is difficult, time consuming and dangerous. A dust cover you do not use while the table is playing is 75% worthless. So, you hinge the dust cover to the turntable’s platform. My favorite design is a 1" thick acrylic base with a fixed upright back higher than the turntable to which the dust cover is hinged. These guys can make it for you. https://www.displaycasej.com/custom-audio-covers
They do excellent work. They made the dust cover for my SME and it is flawless.
Some people here like lewm will tell you that dust covers SOUND TERRIBLE. As long as the dust cover is not attached directly to the structure carrying the platter and tonearm a dust cover will actually make your turntable sound noticeably BETTER. Using a dust cover is just like wearing hearing protection muffs. It attenuates any noise in the room decreasing the turntable’s noise pollution exposure by 20 or more dB. My SME dust cover is 1/4" thick. I played a 1000 kHz test tone by CD and adjusted the volume by meter to 90 dB. Then I put the meter on top of the SME’s platter and closed the dust cover. The volume dropped to 64 dB a 26 dB improvement. It is like putting your turntable in another room.
Next. Any dust on the record while it is playing is detrimental to the sound, record and stylus wear. The stylus generates thousands of volts of static electricity while it is playing which attracts dust like a magnet pulling it right into the groove. The dust cover limits the amount of dust in the vicinity of the static charged record. Along with a dust cover get one of these. https://www.sleevecityusa.com/Antistatic-Record-Cleaning-Arm-p/tac-01.htm This will discharge the record while it is playing and collect any incidental dust at the same time. Records that I purchase new never need cleaning. Since I do not buy used records I do not need a record cleaning machine. I have a Spin Clean for the rare occasion that someone brings a record over to hear but all my close audiophile friends use the same system.

Good Luck,
Mike

Elliot, I have a 9" arm and a 14" arm. It all depends on the cartridge. I put stiff cartridges like the Koetsu and the Air Tight in the long arm. I put softer cartridges like the Lyra, Windfeld Ti and Clearaudio in the 9" arm.
I can't say one sounds better than the other. They sound different probably mostly due to the cartridges. The differences are surprisingly subtle. I can say that I have never heard a Koetsu do bass or sound punchier than it does in the Kuzma 4 point 14. I have a Rosewood Signature Platinum because it weights a lot less than one of the stone models which would push the effective mass over the edge. The Koetsu RSP and 14" Kuzma are a match made in heaven. The Windfeld Ti in the 4 point 9" will track anything you throw at it. It beats all the others in tracking tests. If I were to use just one arm I would get an SME V 12. It is a very light 12" arm and with the addition of head shell weights would be able to accommodate any modern cartridge. I like the 4 point 9 a lot but the reason I have it is that the SOTA will only take 9 to 10" arms. The Tri Planar arms are also excellent.   

Elliot, the poly table Super 12 with a 12" Jelco arm is a great choice as long as you are using a fairly low compliance cartridge certainly less than 15 um/mN. Something like an Ortofon 2M Black will suffer greatly in that arm. The are a lot of MC cartridges below 15 and Sound Smith makes cartridges with very low compliance just for arms like this. 

lewm, you have to be kidding. Please tell me you are kidding. Do you have ghosts in your closet? Even a cheap modern turntable transfers very little energy to the surrounding environment. A good turntable with a good record hold down system that dampens the record with a stiff multi bearing arm set up correctly will transfer virtually nothing. You can place an ear right next to both my tables and you will hear absolutely nothing. To say that this energy bounces off a mechanically isolated dust cover and somehow interferes with the signal is next to ludicrous. How can you possibly compare this to loudspeakers in the same room blaring at 90 dB?  I know you do not like dust covers but you will have to find another reason. You might just say you don't like them instinctively and be done with it. Your loss. Even if somehow a proper dust cover caused some mystical degradation in sound protecting the record is paramount. Not to mention your tonearm. Dust is certainly not a good thing for them either.  The myth about the dust cover being bad was perpetrated by manufacturers that either did not want to supply them or had designs that made integrating a dust cover difficult. Other manufacturers that had to compete on a cost basis were forced into not supplying them further advancing the myth. Marketing and mythology are first cousins.

Elliot, smart man. Measure twice cut once. My contractor had that stenciled on his truck.
The Pioneer arm is in the same ball park as the JVC arm which I like better but I have to admit my preference is instinctive. Most tonearms with removable head shells have higher effective masses. That relatively heavy mechanism is right at the end of the tonearm and adds directly to the effective mass. If you want to use a high compliance cartridge 20u/mN or higher you have to use an arm with a fixed head shell or a novel system like the Kuzma 4 point 9. Look at the Pro-Ject Evo 12. That is a relatively light 12 inch arm. The SME V 12 is another but that one is very pricey. A used Kuzma Stogi Ref 313 might fit in nicely. 

Mike  

Best way to see it is a binocular dissection microscope. You get much better depth of field this way.  

If you have a problem with using a dust cover and it is not just in your head it is because your turntable and/or dust cover are designed incorrectly. In either case you get inferior sound particularly if your turntable is in the room with your system. 
The Dust cover can not be mounted to the platform carrying the platter and/or tonearm. The SOTA Cosmos is an excellent example of a turntable beautifully designed for a dust cover. The dust cover is mounted to the external chassis. The sub chassis is suspended inside with the platter and tonearm mounted to it. The SOTA sounds better with it's dustcover down because all the other sound in the room is attenuated over 20 db. The platter, tonearm and cartridge are better isolated from the environment. With the SME there is no external enclosure to mount a dust cover to so I sit the turntable on a platform to which the dust cover is mounted. The dust cover does not touch the SME.
If you have an unsuspended plinth to which the platter, tone arm and dust cover are mounted the dust cover may transfer some vibration at resonance frequencies to the active parts of the turntable making things sound worse. These tables and designs like the VPIs and Clearaudios need to be set on a platform to which the dust cover is mounted then you will improve the sound with a dust cover and prolong the life of your records, tonearm and cartridge. 
This is obvious and the case unless you hear micro vibrations like lewm.
In which case your system is always going to sound lousy and you might as well give up and play cricket.