i usually did stereo out of the amp to the computer.

so maybe it will be like that to ur board.

plug stereo out on ur amp into the input on the track and start playing.
If you amp has a direct/line/recording out go from that to the inputs on the Tascam.

If not, you'd need to mic your speakers via the Tascam.
Nope; that won't work

You'll have to buy a microphone and mic up your amp.

Not sure if your tascam has preamps; I don't know how those work. But you'll have to mic up the amp
Yes, you'll need to mic it. And for the love of god, theres an edit button for a reason.
Yes, and please don't connect the external speaker jack, you'll end up blowing something in your recorder.
if you can't hook up your amp to it, why does it have a guitar input? Why would you go straight from your guitar to it? There wouldn't be any eq?
thats for your guitar or bass to directly connect.
hooking an external cab. output to anything other than a cab. will destroy both the amp and recorder.

The only way to record your new amp is to mic it..simple as that.
Outboard EQ isn't really important when recording using digital hardware because you can always use editing programs to EQ and add other FX.

for analog recorders, most recorders, like your 414 have a few EQ controls on board so you can EQ before recording to the tape.