sub in bookcase?

I bought the microlabs FC360 speakers and they sound great. Problem is they are huge. Especially the sub. I'm thinking of buying an RCA extension cable and sticking the sub on the bottom shelf of my bookcase. As the cables are too short I can't easily test it out myself.
Is this likely to have a noticeable, negative impact on the sound quality? Typically I will be sitting on a sofa directly across from the bookcase.

It seems i can pick up the RCA phono extension cable for about £2 from ebay. I know I don't need monster, will anything work? I don't know what the monoprice UK equivalent is.

If the bookshelf doesn't rattle as a result, sound quality won't be majorly affected. If it does rattle you may be able to decouple it a bit (Auralex Subdude, speaker spikes, a piece of foam.. something like that) while keeping it in the shelf and get decent results.

Generally cheap cables are fine for audio, but I would make sure to buy a coax cable rather than paired (including twisted pair) wire. Sometimes the connectors themselves on cheap cables will be very low quality that they don't fit well, though.

If you do ever buy higher-end cable, Blue Jeans Cable is the only brand I'd really recommend. But I still don't give them a really high recommendation in general because they are still too pricy. But they are better than Monster, at least. And they're not selling you lies like Pear Audio or Nordost.

Foam isolation I would agree with, but (metal) speaker spikes are not there to isolate vibrations (although an isolating washer may be placed under the spikes themselves). Instead, they generally are there to anchor the speaker or stand in place, especially on carpet, in an attempt to prevent the movement of the cone and coil assemblies from causing movement and vibration of the speaker cabinet due to an insecure or floated footing (Newton's laws and all that).

Anyway, the only additional thing I would look out for is keeping the sub's port clear of obstruction. (Luckily, it looks like that sub is ported to the side, so this should not be too much of an issue. And even if you were to block the port, this would not necessarily make the sub sound worse - just different.) And I would recommend placing a dense foam under the sub for isolation as dandragonrage suggested. (No one wants a rattling bookcase accompanying their sub.)

To be honest, even though directionality is less of a concern with low frequencies, subs can be difficult to place optimally in most rooms. This is why many people do what is known as a "sub crawl" while trying to figure out "proper" sub placement for their space. Heck, your sub may even sound better, to your ears (the only ears that matter), once in the bookshelf.

It will make the bass response less even, since there will be a ton of boundary effects, but if you are happy with how it sounds then there's not an issue.