I believe your preamp gives you volume and blend control. This is a very basic preamp and might be fine for the occasional plug and play. It is not a D.I. though. A D.I. converts your unbalanced high -impedance instrument cable to a balanced low-impedence mic cable. You almost always need this to plug in to a sound system. Some mixers offer instrument inputs but if you are running long cable runs, mic cables are much more resistant to noise so it's better to use a DI box to switch from your instrument cable to mic cable as soon as possible in your signal chain to reduce noise problems. You could potentially get a basic DI box without any EQ options but since you don't have those options on your built in preamp I would highly recommend getting one of the DIs with EQ options. This will allow you to shape your tone and I think notch filters and phase switches are a must for feedback control. Especially having the microphone on your pickup system, you will very likely have feedback issues. I also think having a sweepable mid eq is very important as guitars respond very differently in that mid range so it's nice to be able to have control of what specific mid frequency you are controlling.
Of the devices you mentioned I've only used the PARA DI. It is an excellent device that gives you all the control you need. The Fishman looks interesting with the smooth or compression option (might be an added bonus). I probably wouldn't use the smooth knob but the EQ section looks good. The Acoustimax offers the pedal format, mute switch, and maximizer which can add brilliance if your guitar sounds too mellow. The Ultrasound is lacking in EQ so I wouldn't get it. The extra $20 for the Fishman with more EQ control is well worth it.
Hope this helps.
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