The overload margin is the amount of voltage the phono input can take from the cartridge before it overloads (saturates). The 29 db. reported in the owner's manual is an excellent figure - 20 db. is common with high-end phono stages (the higher, the better).
However, it does not appear that the phono stage in your integrated allows you to vary the load to accommodate the varying electrical parameters of different cartridges (it's set for 47 kOhms, which usually works with MM cartridges, but not always). This is a serious handicap and reason alone for you to look at stand-alone phono stages.
The Dynavector 20X 1mv Special is not a low-output MC. Low output is typically defined as less than 0.5 mv., with 0.3 mv. being considered a typical low-output MC. It's easy to understand why low-output is in theory better - the lower the mass of the coils or magnets, the lower it's output, and the more easily the stylus can move and track the grooves - it's got less mass to move. So, ironically, the lower the output of the cartridge, the more dynamic and "fast" it is. The problem is that the lower the output, the better the phono stage needs to be to prevent noise from contaminating the very low-voltage, delicate signal coming from the cartridge.
A judiciously chosen stand-alone phono stage will almost certainly improve the performance of your analog rig. My advice to you (and to anyone going down this path) is to avoid phono stages that use tubes in the INPUT STAGE. This is the place where the phono stage amplifies and processes the signal from the phono cartridge - even carefully chosen tubes tend to be too noisy for use as the amplification device of a very low-level signal. For this reason, a lot of phono stages (most Audio Research designs, for example) use a hybrid circuit, with transistors (MOS-FET's, typically) on the input and tubes in the output stage.
However, it does not appear that the phono stage in your integrated allows you to vary the load to accommodate the varying electrical parameters of different cartridges (it's set for 47 kOhms, which usually works with MM cartridges, but not always). This is a serious handicap and reason alone for you to look at stand-alone phono stages.
The Dynavector 20X 1mv Special is not a low-output MC. Low output is typically defined as less than 0.5 mv., with 0.3 mv. being considered a typical low-output MC. It's easy to understand why low-output is in theory better - the lower the mass of the coils or magnets, the lower it's output, and the more easily the stylus can move and track the grooves - it's got less mass to move. So, ironically, the lower the output of the cartridge, the more dynamic and "fast" it is. The problem is that the lower the output, the better the phono stage needs to be to prevent noise from contaminating the very low-voltage, delicate signal coming from the cartridge.
A judiciously chosen stand-alone phono stage will almost certainly improve the performance of your analog rig. My advice to you (and to anyone going down this path) is to avoid phono stages that use tubes in the INPUT STAGE. This is the place where the phono stage amplifies and processes the signal from the phono cartridge - even carefully chosen tubes tend to be too noisy for use as the amplification device of a very low-level signal. For this reason, a lot of phono stages (most Audio Research designs, for example) use a hybrid circuit, with transistors (MOS-FET's, typically) on the input and tubes in the output stage.