[Balaji007]

Nothing but, Precompression is necessary. If you are use a precomp,Just try with brute option. What is difference between precomp and precompx?
Ans : Precomp is not a chunk based compression. But precompx is a chunk based compression. Set chunk size in 128 Mb getter good precompression. Plot twist precomp is faster to infalte the files. Precompx was little bit slow.
2. Which external compressor suitable for better compression?
4 types of tools are available. There are
* BWT (Faster to compress)
* CM (Compression and decompression time equal) Ex: MCM,Nz
* Paq (Ultra compression,but time takes more) zpaq,paq8 etc
* lz based compression (lzma,lolz,rz,dlz,xz) Gives better ratio

Every better compreesion based on your Precompression and srep parameters only

3. Why lolz gives high compression ratio?
Because it detects the raw dds and dxt files.

I recommend to use rz or nz. Set rz dictionary size 128m to get better ratio. Also decompression speed is awesome.