I think you definitely need the FBV MkII Shortboard without a doubt.  you can connect your amp to your laptop/PC/Mac or whatever and then use Spider IV Edit to get your sounds dialled in, and then you can use your XT Live as I outlined with the CD/MP3 input - that's the only way you're going to get closest to a power amp scenario with the Spider as it doesn't have an FX loop.   I guess you haven't updated your Spider IV's firmware yet as you don't have an FBV Mk II pedal - when I got mine back in September last year the firmware was at the initial release (1.02 I think) and I thought the sound was generally quite gritty and tinny compared the the Spider III 150  which I still have.  I A-B'd the two amps using factory presets of the same name and felt that the Spider III sounded better at first.  After I updated the Spider IV firmware to 1.04 (I think) the amp did sound better and then better still with the latest updates (1.51/1.52).  Bottom Line is IMO anyway - the Spider IV is a solid state amp and it doesn't have the b*lls of a valve amp but it is nevertheless a good sounding amp and very loud too  - and especially for the money  

I could never get on with my X3 Live with a traditional guitar amp - fine with going into a flat EQ desk/powered speakers/PA or whatever but not into the power amp section of a Marshall - too much like hard work to get anywhere near a decent sound.  My XT Live however sounds great into a traditional Marshall amp's power amp section.

The XT Live and Spider III I believe are based on older POD 2.0 DSP technology, whilst the Spider IV and Spider Valve MkII are based on X3 Live DSP technology and I still have a liking for the POD 2..0 style sound, so I understand why you say the Spider IV 150 amp tones seem to be a bit lacking.  If you update the amp's firmware and use Spider IV Edit though, I think you will be able to get some killer tones out of the amp as it stands and I do believe that it is capable of more tonal variety than the older Spider III and the XT Live - but it will take some work to get there

I am trying to connect my pod x3 live to my spider IV amp via the cd/mp3 port without luck. I have a dud jack in the front guitar port and still no luck.  Am I missing something?