I've searched for Q Acoustics 220 / 220i but come up emptyhanded regarding specs or reviews. The closest I get is the 2020i currently selling at £129/pr. Am I missing something here or is 220i a typo for 2020i?
If it is the 2020i with the 2070 sub then I don't see this as a straightforward step up. In some respects it would be a downgrade.
The 734s were £500 floorstanders, and at the time £500 was about the tipping point where it was possible to make reasonably-stiff large boxes so that it was worth buying floorstanders. At £400 and below the better option was larger stand-mounted speakers. They had tighter and more precise bass than similarly-prices budget floorstanders.
Like most larger speakers though, the Missions need some space around them if they're to perform well. Put them too close to a corner and what should be deep controlled bass becomes boomy and overbearing. Small speakers are more forgiving in this respect, but they lack the richness of tone and depth of their larger cousins.
A small pair of bookshelfs and a sub seems like the logical solution; the main speakers are more forgiving of placement, and the sub allows the bass level to be controlled independently, but there's only so far that this combo can be stretched before the underlying limitations are exposed. Reviews which sing the main-speaker's praises do so in the context of similar budget-oriented speakers and not against competition at 3-4 times the price and significantly larger.
I'm reminded of the first time I listened to Monitor Audio BR2 standmounters versus the BR5 floorstanders. On their own, the BR2s sounded great and it was difficult to think why stepping up to a floorstander at more than twice the price would be needed. Then I heard the BR5s. It wasn't just the extra bass extension but what the speaker added to the midrange too. Things that the BR2 had sketched in got the full colour treatment with the BR5s. Whether it was musical instruments or voices, they all had more timbre with the bigger speakers. Stepping up to the Silver bookshelfs repeated the same trick despite being physically smaller than the BR5 floorstanders. Then the Silver flooranders repeated the same trick yet again. The 2020i isn't Q Acoustics top-of-the range standmount speaker. There's a reason why the 3000-series and the Concept-series both exist.
This brings us to the sub. There's a reason that the 2070 sub looks the way it does with its twin 6.5" drivers. The primary concern is having something easy to hide. Having two smaller drivers compared to one larger driver won't make it play as deep. If the amplifier is powerful enough then it might play as loud as a single larger driver, but it won't reach as low down.
Depending on the main speaker model, the package isn't bad value. You might even be able to buy, try and then move on for what you paid, so if you can afford it then give it a whirl. It's a shame the tweeters on your Missions are bust. Doing a proper A/B comparison would be interesting.
