Everybody steps into the recording studio wish to attain the highest standard. Atleast that is their initial motive before they get too drunk and too drugged.
I really see no trace of underpower in your system. The 508 should have enough gain behind it. I personally witness the A300 sounded more vivic than the Plinius SA100MK2 which is a Class A 100 watter. As for the VA Bach, I really hope that I'm wrong, is not the tightest and fastest speakers I've heard. To appreciate music on the heavier side you need these 2 characters.
Tightness is important. No tightness no attack. I say this not because it wasn't named after the S Bach from Skid Row.
Tightness doesn't apply only to bass but also the low mid and mid where most of the guitar chords were played in this region. You need punchy guitar, it may sounds strange but I know I'm using the right word. You may call it attack but in the studio, you can feel or hear the rhythm guitar sound punching out from the amp.
To find that out,it is easy if you have Green Day's "When I come around". Play the opening passage of the guitar intro before the drums and the rest kick in. Chords in this passage were played on the lower frets of the guitar, if your speakers are too mellow or loose for high current music,you'll hear some "low-mid boom" or close miking effect. This range is between high fret bass and low fret guitar. you'll thus have problem telling whether the sound is of a guitar or a bass.
Speed needs to be determined by low bass. Highs are always fast. Listen to the basslines of Blood, Sugar, Sex, Magik from RHCP. If you can hear the bass drum and the fast bass guitar in clear seperation, you'll really love this song. And your speakers are doing a fine jod. Remember, bass guitar decay slower than the bass kick drum, be sure you can tell which is which. Than move to Iron Maiden and enjoy Steve Harris. For Highs, check out drummers who love to hit high knobs, Cozy Powell of Rainbow is one of them and most of the rock fans have one of 2 album of them. Listen and identify the difference between high knobs and the rim of the cymbal, high knobs shouldn't sound splashy. The rim should fade in zzzzziiiing rather then shhhhh. Don't get too excited with all the ting ting ting, these are all percussing pieces, be sure that they are hit but not like wind chimes. Listen to Time(Pink Floyd). Listen to the alarm clocks. See if any one of these alarm clock hurt your ears, not one of them should if your set up is accurate.
For Vocal, check out Scorpions, Klaus Meiner sounds really bright if your system is thin sounding and Meatloaf will sound too young for his age(he needs to sound tight and a bit congested). Listen to Bruce Dickinson for the punch, again vocal too have punch, BD has got powerful lung, his voice is not too high but very punchy and shouting on your face . Forward, gore but never metalic and Zing Zing. Female, If you know Stevie Nick, use her. Her voice is the easiest to remember. Or your wife if you have a cassette deck.I'm serious, you do the recording and the listening. Ask her to sing and talk. If the voice is accurate during playback, than the vocal department should be alright.
Check out the tightness of your speakers first. By GreenDay and RHCP if you have them. Not that I trust the Meridian or MF more than VA, they all are good stuffs and from Europe. Just becuase the speakers, the "vocalist" of your set-up have to be the right guy. Don't ask Nat King Cole to sing SUM41.
Sorry for this long one, I just wish that rock materials can be appreciated more in the audiophile circle. Good Luck.
