12V Deep Cycle battery with Pure Sinewave inverter for tube amp?

If there is a danger with fumes releasing, would a gel type battery solve this problem?

Yes, Sealed Lead Acid type of battery is the answer! However,  Gel cell batteries must be charged correctly or they will permanently reduces capacity.

I used a 100AH Odyssey PC2150 deep cycle battery to power my modified Sony SCD-1.  They have vanishingly low impedance, which is a function both of their design and high AH capacity.  It's good to add a large amount of capacitance between the terminals-- particularly using an array of parallel capacitors.  This filters noise and improves transient response, even into low current loads.  Keep the battery cables to the inverter thick and short.   

I've also got a cheap furman power conditioner. Would connecting this along with the inverter benefit any positives, or should I just go straight into the inverter?

I am converting from DC to AC. My amplifier is a 115v AC amp, thus the conversion to AC must be done. Along with capacitors @dgarretson , any link you could send me on how this might be achieved would be greatly appreciated. I have no knowledge at all when it comes to electricity. Especially capacitors(had to Google what they were).