- Dual doesn't use separate ground, iirc it's 'embedded' on right channel ground. However, at normal listening volume, any hiss or hum shouldn't be very noticable - if it is relatively loud then you may need to check the wiring. One of Dual's weakness is the headshell connector, contact spray will help a bit there.
- Separate phono preamps are mostly sold to be used with current breed of amps that only have line level inputs, although they do present slight improvement over old designs. I believe this would be an overkill for a 1218, just use the built in phono preamp.
- Shure M9x is quite decent, but maybe about time for a replacement. Audio technica AT-110 or 120 should give good value for their performance. If this Shure is a model with brush attached then you may need to put higher tracking force, about 1.75gr with brush engaged. It's better to err on the heavy side rather than having not enough stylus pressure.
- Awfully sorry to say that it might be difficult to get good sound from 1218 as (again iirc) it uses rubber idler to turn the platter which now may harden with time and affect the speed stability. Please check if it's belt driven or not. Ime going from idler Duals to belt driven ancient thorens td 165 (which can also be had for almost free thesedays) was a revelation.
So in order to bring the best out of your table I believe a good cartridge will be the optimum solution. Don't forget to clean your stylus and records as wonderful sound of vinyl won't happen with dirty needle dirty LPs and old recordings anyway. Once you decided to get more into vinyl, take a look at various old thorens on ebay, the simple models such as 14x/16x are almost indestructible even by plugging into wrong voltage.