Don't expect to get a perfect high gloss mirror finish without spray equipment and lots of experience.
But a very nice gloss finish with a brush? It can be done. I get excellent results with Waterlox.
I wouldn't worry about filling the pores or using a sealer,you should sand after each step and being a novice you risk sanding through the veneer. The less steps the better.
1.) Sand veneer lightly after glue up. You can do a wipe down with mineral spirits to see if you missed any glue squeeze out.
2.) Brush a coat of Waterlox on all 6 sides, let soak in then wipe off excess, let dry overnight.
3.) Sand lightly, and remove dust with a tack cloth.
4.) Vacuum your entire shop or finish in a clean area.
5.) Lay the speaker on a side, you will be finishing one side/day.
6.) Use the tack cloth again.
7.) Brush a thin coat of Waterlox on the side facing up, it will flow together with no brush marks.
8.) Watch for drips on the edges and carefully wipe off with a clean cloth.
9.) Repeat on the other three sides, top and bottom. Always use a tack rag before applying the finish.
10.) When all sides are done, sand lightly, vacuum again and do it all over again.
11.) 3 coats will look pretty good.
Dust is your enemy. Pour just enough Waterlox in a clean container to do the one side, then discard the excess. Do not pour back into the container, it will contaminate the can. Buy a new brush, Purdy is a very good brand and will not leave little broken fibers in the finish.
I you decide to use varnish, strain the varnish through pantyhose first.
Wayne